Player physics can be modified using physics overrides. Physics overrides can set the walking speed, jump speed and gravity constants. Physics overrides are set on a player by player basis, and are multipliers - a value of 2 for gravity would make gravity twice as strong.
Here is an example which adds an antigravity command, which puts the caller in low G:
player:set_physics_override() is given a table of overrides.
According to lua_api.txt, these can be:
The sneak glitch allows the player to climb an ‘elevator’ made out of a certain placement of blocks by sneaking (pressing shift) and pressing space to ascend. It was originally a bug in Minetest, but was kept as it is used on many servers to get to higher levels. They added the option above so you can disable it.
Please be warned that mods that override the same physics values of a player tend to be incompatible with each other. When setting an override, it overwrites any overrides that have been set before, by your or anyone else’s mod.