Because of newton third law of motion which states that every action has equal and opposite reaction that we we pushes something it also pushes us back with an equal amount of force jet planes also works in this principle like a balloon flies away when we vaccum all air from inside it
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it is because of the newtons third law of motion i.e.every action has equal and opposite reaction while jumping from a boat man pushes a boat in backward direction .This is action and due to this action reaction of equal magnitude in forward direction is producedhence a man jumps from a boat with this force of reaction
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(a) When we push our foot against the ground backwards(action) the ground exerts an equal and opposite force(reaction) on our foot which causes us to move forward(Newton's Third Law of Motion).

(b) When a man steps ashore from a boat, he pushes the boat in a backward direction because the man needs a force of reaction to step out of the boat for which he exerts a force(action) on the boat.

(c) When someone fires a bullet from a gun a force is exerted on the bullet( action) and the gun experiences a recoil(reaction).

(d) Jet airways have a streamline shape which helps to reduce friction with air and body of the jet thus reducing the friction and cutting through the air.

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