first off the hand over slide way of racking isn't considered a gross motor skill. it's a fine motor skill, just like hitting the slide stop/release is, or pulling the trigger is, or grabbing the gun is. Punching, kicking are gross motor skills. anytime you grasp something, or manipulate something it's a fine motor control skill.
secondly, the slingshot method refers to grasping the slide with your weak hand, gripping the rear of the slide between the thumb and index finger, and pulling back and up, until the slides stops and your hand slips off allowing the slide to go forward into battery. This is the method typically taught to people using Beretta's or Baby Eagles or other guns that have slide mounted safeties. If you try the "hand over" method, you will often inadvertently set your safety into the safe position. using the slingshot method you will not as your fingers and thumb will put upward pressure on the safeties. (one reason I went with a G model Beretta, it didn't matter how I cleared a malf, the safeties were under spring pressure to stay up).
the tap rack drill is also often taught using the slingshot for people using the above mentioned guns, but sometimes isn't. Because it is weaker grip on the gun, and if it is truly jammed, you need all the strength you can get to work it.
Thirdly, muscle memory. well, I'll simplify it. Some people argue training one method (for instance hand over rack) for everything, from dropping the slide, to initial rack, to clearing. And the argument is that you will remember this easier. bull####. if that's the case your training sucks. because then how will you learn to apply the slide lock whilst locking the slide back to clear a stage 3 malf? or how will you learn the intricacies of sight alignment, or proper trigger pull and follow through?
train the way you want to train, one method for each type of situation, or train with one for all, the only thing that matters is training. learn how to do it, learn how to tell what's going on with your gun, and to identify problems as they occur.
I use the slide release, it's faster, allows me to stay on target better, and doesn't chew the #### out of my hands on guns with target sights. When shooting my Glock, I'll go hand over if I miss their horrible little release, and with my Beretta I'll use the release, and slingshot on a malf, or a hand over if I default that way.