Hax Mode is a building technique discovered by Scrap Daddy (in collaboration with Sage) that allows one to stack anything into everything. The stacking effects it allows are similar to what is possible with bot file editing and AAM, but unlike these, it can be done entirely in-game, potentially making it a legal glitch. Because of this, Sage, and Scrap Daddy were reluctant to release it to the public for fear of the controversy it would cause, even going so far as to say that it would be "the end of stock". They did eventually release it, but the community quickly decided that most uses of it would not be considered legal for standard tournaments due to their being virtually indistinguishable from AAM.
Instructions for activating Hax Mode can be found here