In order to take off:
1, There must be a force (push)
(Newton’s First Law – change requires a force)
2, The force must be enough to accelerate the horse’s mass. Think about trying to throw the horse up there yourself: it’s the change in speed and the mass of the horse that matter.
(Newton’s Second Law, Force=mass x acceleration)
3, The floor must resist the push, or the horse will just sink.
(Newton’s Third Law, equal and opposite actions)
In our picture our rider is ahead of the horse, and lifting with his hands.
As he’s ahead of the horse his centre of mass is in front of the horse’s centre of mass. This prevents the horse from raising its front end effectively (and puts him at risk of a fall). Likely outcome: pole down in front.
However he’s trying to compensate by lifting with his hands. This shortens the horse’s neck, bringing the head in. As the head and neck are a major part of the horse’s weight, this shifts the centre of mass backwards (caudally). This could help avoid the pole in take-off BUT also has implications for the rest of the jump. The head and neck act as a counterbalance over the fence, rotating the back end up. The longer the lever of the neck the more turning effect will apply to the horse (Moment =force x distance) so the higher the back end will go. A shortened neck prevents this adn lowers the back end. Likely outcome: pole down behind.
So which was it in the case of this rider? I’ll leave you to decide.