Jumping on a monster usually does 1 point of damage for low jumps, with fractions of a point extra damage done based on falling height (for example, 2 points total are done when falling from a height that would normally cause the character to take damage). When using the spike shoes, damage is tripled.
Note that it's sometimes unintuitive what happens when your feet come into contact with enemies:
- If an enemy jumps up into your feet, and you're stationary (i.e. standing on a ledge, or clinging to a ladder, rope, tree or wall), then the enemy does damage to you, and you don't damage it.
- If you jump up and collide with an enemy mid-air, and you hit it with your head, then the enemy damages you, and you usually (but not always?) do damage to it.
- If you are immersed in water, you cannot do jumping damage to enemies and will instead take damage (presumably, you fall too slowly to damage them). This only applies after you are fully immersed, however - for instance, it's possible to jump on a piranha and kill it without taking damage, so long as the piranha is in the top layer of water.