Make your center of gravity be above your feet.

[Summary]
In the way of type 1, stretching both hip joint and knee joint at the same time and jump alike usual jump.

This jump with shifting CG initiates rotation.
(cf. To arch your back makes rotation but decreases height)

If you hurry to tuck or pike, your hip joint will start being closed before taking off.
To get the utmost power from the ground, your knees and waist should be extended while pushing the ground.
And you should make your knees and hip joint straight at the moment of taking off.
Shift your CG by hollowing (or arching for a back flip) chest, not by piking (arching) waist.
Also you can prevent backward rotation when doing a front flip.
(cf. Keep your torso straight and stick your hips out)

He arches his chest and his knees and waist are straight and vertical to the ground.
(You can stop/start videos by pushing space key.)
Therefore, he can jump with the utmost power.
However, it requires flexibility of chest.

The same is true in a standing front flip.

[Details and Theory]
When it comes to flip from standing position, slight leaning  forward or back is necessary, because a force by pushing the ground must be applied  not through the direction of CG.
However, if you lean back too forward or too back, your power of pushing the ground will be decreased.
Because, you jump by opening your knee joints and hip joint.
You must open both joints but if you lean too much, your hip joint have started closing after your hip joint passed the straight.
The ideal posture of  jumping is the posture in which your waist and knees are straight and vertical and you lean forward or back with your hollow(arched) chest.