The pairing is called complex or contrast training: a heavy back squat triple at 85 to 90 percent, rest 3 to 5 minutes, then 5 box jumps. The jumps are measurably higher than they would have been without the squat.
Timing is the whole trick. Too soon and fatigue from the heavy set dominates; too late and the effect has gone. Three to five minutes is the usual window, and better-trained lifters both potentiate more and need slightly longer.
It is for athletes with a power goal and a decent strength base. A client who squats less than about 1.5 times bodyweight gets little from it and should keep building the squat.