Zet Box is a manipulation of the boxes after the guns to save a gravity cycle in that section. Instead of pushing a box past the gravity switch, waiting out the cycle, and setting it up the slope in order to get to the upper area with the box, it's possible to push the button box onto the trigger and climb the rising box such that both Limboy and the box make it up to the upper area in one cycle. While this saves about 5 seconds in reference to the area, it doesn't necessarily save that much time due to how Piston Cycles work.