I've had a number of questions regarding the correct performance of the box squat such that I wanted to address it here. The main theme being "When performing box squats, should one sit on the box (apply all the weight to the box) before coming back up?"
I love box squats and coach them often in my clients. We should probably be doing some form of box squatting every 7-10 days to reinforce mechanics. It's an excellent tactile cue and external focus for the posterior chain allowing one to sit deeper into the squat and teaches the athlete how to maintain vertical tibias. I also find it as a useful progression of the hip hinge pattern, as well as a way to standardize depth of squatting for training noobs and/or groups.
In theory, the purpose of sitting on the box is to challenge the concentric drive out of the hole by reducing the elastic response (stretch-shortening) of the system during the transition from eccentric to concentric phases at the bottom of a traditional squat.
I understand that and share the goal to increase power out of the squat. But fully sitting on the box with a large external load on the spine is not a good idea:
1) It essentially forces a posterior pelvic tilt (or butt wink) in which the spine goes from a neutral position into a flexed position while the system come off of tension — this is nightmarish for spine and hip integrity. You're essentially relying on the passive structures of the lumbopelvis (IV disks, longitudinal ligaments, SI joints) rather than neuromuscular control — not exactly the goal of squatting.
2) In order to reclaim the tension needed to come off of sitting on the box, I have seen far too many generate that stability thru the spine by rocking forward and locking into hyperextension: a stable position, certainly, but not a sustainable one.
3) So we've essentially gone from a braced neutral spine (assuming set-up was correct) to an unbraced and flexed spine to an extended spine over the course of the movement — which is the opposite of the goal of squatting which is to maintain neutral spine while generating wicked force thru the lower extremities. I think we should absolutely challenge all spinal shapes for maximal competency and resiliency, just not during box squats with heavy weights on our backs.
So to correctly and safely perform the box squats, I coach my clients to touch and go.
1) Sit back into the posterior chain (with appropriate load ordering) and pull into the depth until the ischial tuberosities (sit bones) make contact with the box.
2) Once that tactile cue is received, rocket out of the bottom position to generate power out of the hole.
If you want to have the athlete pause for a beat, that's cool, just don't have them sit down. That isometric pause is essentially the ti-phasic training that Cal Dietz promotes, NOT completely coming off tension by sitting down. If you or your athlete is really obsessed with removing the stretch-shortening cycle from the squat (rarely is this sport appropriate, by the way, as even in jumping we dip before we drive to claim an elastic response), set the rack at the bottom depth desired and just have them squat it up from the bottom. This maintains braced neutral without loss of lumbopelvic position in the hole.
Bottomline: we love box squats and do them often — just don't sit on the box, bro.