Pressing X will let you respawn at the starting point or at a checkpoint when activated (this costs one life).

Respawning keeps all collectibles in your inventory.

Respawning also respawns all keys.

Bonus levels can be used to collect lives (which can then be used with X to respawn).

Avoid collecting clocks as they add 15s to the bonus level.

Moving in the opposite direction removes all velocity of the former direction.

Hitbox detection isn't perfect, so sometimes jumping an enemy from below also kills them.