Pull the lead hand back, sit on the lead leg, then shoot the uppercut
The small detail in this clip is the lead hand.
Before the uppercut comes out, the lead hand pulls slightly away from the guard while the body slips off the line. It is not a lazy hand drop. It is a quick chamber.
At the same time, the weight loads onto the lead leg.
That is what makes the uppercut come out sharper. You are not standing tall, slipping, then trying to lift the punch with your arm. You are already sitting into the shot before it fires.
The sequence is simple:
Slip off the line.
Pull the lead hand back just enough to load it.
Keep the other hand home.
Sit into the lead leg.
Drive the lead uppercut up short through the middle.
The important part is keeping it compact. If the hand drops too low or the punch loops, you are open to getting countered. In the clip, the punch stays tight and comes right back through the center.