Nothing in Mach itself is an 'Emergency Stop'. An Emergency Stop generally removes all power form the things that move and must work irrespective of any software, or the PC itself working. The Stop button in Mach is just that, a Stop now button. It ceases all movement immediately (so you can loose position.) Think of it like Mach having a Play, Pause, and Stop buttons like on a CD player. Pressing stop, may not in some cases, stop a malfunctioning CD player but pulling the power cord from the wall will. An Emergency Stop is like pulling the power cord.