Watch the video and work out where the centre of mass is throughout the whole video and the direction of the force of the bed.
To create rotation (angular momentum) you need to create an off-centre force.
In this skill the front somersault (piked) it looks as through the shoulders are being brought forward, however it is the hips that are displaced upwards and backwards at take off. This changes the position of the centre of mass.
The vertical force of the bed continues to go upwards which does not go directly through the centre of mass as it’s position has moved forwards slightly.
There is also a horizontal force that is applied by the feet pushing backwards into the bed during take off that causes the body to rotate about the centre of mass.
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The pupils body shape may change during the flight phase which will alter inertia resulting in the rotation speeding up or slowing down but no additional momentum can be created whilst in the air