Here is an example of the death warp:

All that is required for this glitch is a place to die. At the same time that you die, press R to reset the level. If done with the correct timing, the player will warp to one block underneath the top-right block. Sometimes this may skip an entire level (as shown), sometimes it may only skip one part (such as in the "The Wind Tunnel" section of "Fan-Tastic"), sometimes it allows the player to clip into another part of the level early, and sometimes it merely traps the player.

A similar glitch can be done if the player hits the R button twice in quick succession at the first level select area. This will allow the player to visit the second set of levels earlier than usual.

This only works on versions where you can tap R twice to reset the level. For versions where you have to hold R to reset the level, you have to time the death perfectly with the reset, though this gives you a very precise window to die, making In The Mix skip (and many others) impossible. Therefore armorgames should be used over kongregate.