Keep your torso and shins should be parallel

While your feet are on the ground, your torso and shins should have been parallel to jump with more power.
And get more height.
(Just before takeoff, you don’t have to make your torso and shins parallel to shift your CG, though.)

Front flips are the same.

It is almost the same as How to stretch out your knee joint when taking off.
The objective of both of two tips is to push the ground with more power.
But  this tip is more difficult.
Because if your torso and shins have been parallel, your knees will be extended up but we cannot affirm the converse ; even if your knees are extended up, it doesn’t mean that your torso and shins were parallel.

Typical case is that because you want to initiate rotation by swinging your upper body, your torso leans back too early and  the two parts become not parallel.
You should only  intend to jump with the maximum power and just before the takeoff, arches your back slightly.