The M2 X-Strap is primarily used for TORSO SIDE CRUNCH & SIDE TWIST and GIANT CRUNCH. The TORSO SIDE CRUNCH & SIDE TWISTS are among the top 5 best exercises in the world to strengthen, tighten, and tone the obliques and waist muscles. If you don't want a smaller and tighter waist, do not get the X-strap. ;) 

Only compatible with M2 models, including: M2, M2S