Out of the countless Smith machine lat exercises, I'd have to say that pull ups are easily the most avoided.
Put simply, the Smith machine pull up, though assisted, is still ridiculously challenging for most people. So rather than rising to the challenge, they refuse to even participate. Opting instead for cushy alternatives like lat pulldowns and—worse still—the assisted pull up machine.
Pinning your hopes on the assisted pull up machine for getting stronger at real pull ups is like lowering the hoop to get better at basketball—you might improve at the game you're playing, but you'll never progress at the real thing.
Conversely, performing Smith machine pull ups is the most reliable way to fast-track your bodyweight back strength. And you'll soon find out why.
Rather than merely pulling yourself up, think about actively driving your elbows down to the ground.
This form cue alone will turbo-charge your pull up progress because (1) it encourages you to maintain proper posture—thus preventing the exercise from turning into a poor man's inverted row—and (2) it recruits your biggest and strongest pulling muscles—your lats and traps.
In other words, your hands are pure and simple hooks, and your back is the powerful machinery that actually does the heavy lifting.
Grip the Smith machine pull up bar tightly. Grip it like a vice. Doing so radiates tension down the entire kinetic chain—to your other muscles—and makes you stronger. Try it for yourself. Discovering how a firm grip can boost your pull up proficiency is positively eye-opening.
When a commentator says that a boxer "has got his legs back under him", they mean that the fighter has regained his strength and balance. Well, the same holds true for you and me when we perform Smith machine incline pull ups.
By keeping our feet underneath our centre of gravity (remember, we're letting the bar stretch our lats), we can make the exercise easier by using more of our leg muscles for assistance. This customisation is just brilliant if you're a beginner because you can acclimatise your body to the feeling of pull-ups without having to lift your entire body weight from day one.
Then, once you become stronger, you can make the movement more challenging by placing your feet further away from your body. Or, to put it another way, you're decreasing your knee bend to limit the assistance from your quadriceps.
Control the eccentric. And by that, I mean control the lowering phase of the rep (no need to restrain the quarter-squatting lunatic). Doing so provides your back muscles with more time under tension, making them grow bigger and stronger in less time. And a stronger back enables you to perform more pull ups. So always control the eccentric—only a lunatic wouldn't.
I'm a minimalist when it comes to training—and even more of a minimalist when it comes to training accessories.
However, I don't perform bodyweight training without my trusty magnesium carbonate—my indispensable liquid chalk.
This particular liquid chalk dries in seconds, and it easily washes off with only a small amount of soap and water. More so than any supplement, this chalk increases my pull up strength because it keeps my hands free from sweat, which allows me to maintain an unwaveringly firm grip on the bar.
Gym chalk is one of the cheapest yet most impactful investments that you can make in your bodyweight strength training journey.
Just make sure to go with the inconspicuous liquid variety if you train in a Globo gym. Otherwise, the gym staff might give you a mind-numbing lesson on why they don't want their sweat-ridden equipment to become dirtied by chalk.
It wasn't until I started training with the Captains of Crush Hand Grippers that I realised how much my grip strength was holding back my pull up progress.
I always had decently strong lats from doing pulldowns (with straps, of course). But it never occurred to me that my hands and forearms needed direct work. So I decided to ditch the straps...
And as soon as I did, it felt like some invisible force had sapped all of my strength and vitality. Had my back suddenly got weaker? No. I was simply masking my deficit by using lifting straps!
Finally, I decided to get serious about grip training. I was sick of being "that guy" who could almost max out the pulldown machine but who could barely get 5 pull ups.
I ordered four Captains of Crush Hand Grippers and got to work. At first, I could barely close the CoC Sport (the 3rd lightest out of 11 grippers!), and my ego was feeling the heat. However, I persevered. And as of today, I can close the CoC No.1 for 10 clean reps. Not amazing by any means, but it's good for me. And my pull up strength has already shot through the roof.
I did an AMRAP test (as many reps as possible) and got 15!
So that's an increase of 10 reps in 60 days—all by working on my grip strength and bumping up my pull up frequency from once to twice a week.
I've also used the FitBeast Hand Grip Strengthener.
It produces between 10kg-60kg of resistance, and you can adjust the tension in a few seconds by turning the small knob. Of course, it doesn't compare to the quality of a Captains of Crush grip trainer. However, if you're on a budget, it's a great choice to increase your grip strength for pull ups.
Your lats are the prime movers during Smith machine pull ups because they're the muscles responsible for closing the distance between your torso and your arms. In other words, your lats are shoulder extensors .
Your traps are highly active whenever you perform pull ups using Smith machine systems because they have to stabilise your scapula so that you can properly engage your back muscles.
If you want to build up your beach muscles while perfecting your pull ups, using an underhand grip will grant you your wish by increasing your biceps activation. You've probably heard of this pull up variation before—it's called the chin up.
The brachioradialis is a powerful elbow flexor that receives significant stimulation whenever you perform overhand pull ups.
Along with your small hand muscles that only scientists care to remember the names of, your forearm flexors are responsible for helping (or hindering) your pull up progress by connecting your body to the bar (your hands are hooks, remember?).
You'll only grow from pull ups if you can do them. And, since most people can't, performing pull ups on Smith machine stations is a brilliant way to build your back without having to resort to the dreaded assisted pull up machine (more on this in a sec). 
If you perform enough Smith machine pull ups, and also throw in some assistance exercises like lat pulldowns, then you might be surprised to discover that you can do 3, 5—even 10 perfect pull ups.
This is because Smith machine pull ups are strikingly similar to the real thing.
With the assisted pull up machine, however, all of the stabilisation is handled for you. So while you might build some superficial muscle, you're not developing the crucial stabiliser muscles that you need to do proper bodyweight pull ups.
Hence why many so-called "bodybuilders" can max out the lat pulldown machine but can't do a single bodyweight pull up!
If you work a desk job like me, then you probably don't exactly have military posture. In fact, perhaps your posture has regressed into a permanent slouch. Whatever the current state of your spinal health, Smith machine pull ups (and back training in general) can help by giving you better control over your scapula and thoracic spine .
If you catch yourself sliding into a stoop, then you can simply engage you back muscles (just like in a pull up!) to straighten your torso and reduce the burden on your lower back and neck.
Bodyweight rows tax your traps and get you used to pulling with your back muscles rather than with your arms. They're single-handedly my favourite pull up assistance exercise.
The assisted pull up machine is a decent piece of equipment for building your back because you can focus purely on the working muscles—you don't have to stabilise yourself. However, this ease of use also makes the machine terrible for getting stronger at real pull ups because your muscles don't get used to stabilising your body weight .
A lat pulldown machine is an excellent tool for increasing your pull up strength—if you use it correctly.
Since the hardest part of a pull up is touching your chest to the bar, focus on pulling the cable attachment all the way to your chest. This way, the exercise will have the maximum carryover to pull up training.