As you see in step 1 she is completely still (at rest) and she will stay still until force is applied then she will be in a constant speed doing back handsprings (staying in motion) until force is applied to stop herself.
As you see in step 1 they are applying no force to the flier (the object) so the equal amount of force is being shown (she is not going anywhere), but then in step 2 they are applying force and the same amount of force is being shown (she is now "fling in the air"). And the amount of force they are allying all depends on her weight (mass).
In a stunt as the bases and back spot push the flier up they are also pushing them self up off of the bases shoulders so as we are exerting force to get them in the air they are exerting the same amount of force to help us hold them in the air . Also as we are pushing up on her feet she is pushing down on the bases shoulders so they are exerting force in the opposite direction