Unfortunately since I'm on vacation I don't have the best facilities to answer your question, but hopefully I can give you a good clue. AlterYVelocity is a key piece. But if you're adding a new rule to a sprite that already exhibits gravity, you might be going about it in the wrong way. You should find an already-existing rule that calls AlterYVelocity, and then you should be able to adjust its parameter either higher or lower to affect jump height. Alternatively you could find a rule (it might be called Jump) that sets the dy property of the sprite, which determines how hard the sprite jumps when a jump starts. You could also adjust that in either direction.