The missile contains an accelerometer that measures deceleration. In other words, how fast the missile stops once it hits the surface. A numerical value (most commonly referred to as G-max) is then generated. A high G-max value indicates the missile stopped quickly. Think of this as dropping the missile onto concrete. If the missile was dropped onto a pillow, it would take a longer time for the missile to stop and the softer surface would produce a lower G-max value.