coach probably doesn't understand the basics of physics
Box squats being a prime example of his sillyness
Get a bar on your back and prepare to squat, you go down into a squat position with every fibre of your musculature supporting your skeletal structure then what do you do, you sit on a box totally relaxing your quads glutes and lower back, the weight still on the shoulders then bears down on the spine, the weakest area (L4 & L5 joints) take all the strain.
Ideal way to blow out a disc.