This could be presented as a FRONT vs BACK on the proper technique for a lat pulldown exercise but theres really no point. SO here’s 3 Reasons Why You Should STOP Posterior Lat Pulldowns.
  1. Risk of Disc Herniation. There’s a reason it is so important to maintain neutral alignment of the entire spine during ALL weight bearing exercises. It’s the potential risk of injury to the vertebral disc. In this case, we are looking at the risk to the cervical spine. Placement of the lat pulldown to the posterior side means flexion of the neck during the loaded(and unloaded) phase of the exercise, this puts pressure on the discs towards bulge which can lead to disc herniation which can only be repaired by major surgery. As you’ll later learn specifics, it’s not worth it!
  1. Risk of Rotator Cuff Damage. A lat pull down requires nearly maxing range of motion of external rotation of the shoulder for most people, men especially; the exercise is loaded through a cable which requires more expert control throughout said range of motion. Moving the angle of resistance to accommodate being  behind the head means putting more pressure and work for the rotator cuff, often  times beyond the functional design of those muscles. 
  1. Science says its not worth it. A study performed back in 2002 shows there is no additional muscle activation of the latissimus dorsi(lat) by taking the exercise behind the head. Based on biomechanics taking this exercise behind the neck would possibly activate a bit more posterior deltoid(delt) but there are much safer options to perform this. This study involved testing muscle activation of this exercise from three different positions, wide grip in front won. 

How to do it right. 

  1. Choose a weight that makes sense for you. I cant tell you how often I see butts come completely off the seat to accommodate pulling more weight. I have no idea why people do this, other than to say they could…ish. It’s no longer a lat pull down if you’re having to incorporate different muscles in order to complete this. Choose a resistance that allows you to complete the exercise without butt lift or back extension to perform. 
  1. Core engaged. This comes first in any and every exercise or at least it should! This will also help prevent you from hyperextending while performing your exercise, proper breathing techniques will assist your body in proper core engagement which for this exercise would be exhaling as you pull and inhaling as you release the weight back towards the pulley. Start the exercise from a position that is just slightly hinged back(lean back) from the hips.
  1. Wide grip, elbows in line with your trunk/flank. The wide grip with the weight bar coming in front of you is the standard way to perform a lat pulldown and also confirmed to have the best muscle activation. If your elbows flare out or back which is very common it is no longer a lat pull down exercise. If you can not maintain control and alignment of the exercise return back to Number 1. And start over. 
  1. Signorile, J. E., Zink, A. J., & Szwed, S. P. (2002). A comparative electromyographical investigation of muscle utilization patterns using various hand positions during the lat pull-down. The Journal of Strength & Conditioning Research, 16(4), 539-546.