I learned this spotting method from Michael Manzanet at BGS, for spotting chin-ups/pull-ups.
Open hand squeezed between the legs by the gymnast. It helps the gymnast maintain form (like squeezing a beanbag); and the gymnast only gets an assist if she needs it by squeezing the spotter's hand.
This was just a quick take (and the coach in the video is new to this. The spotter does not grab the gymnast. The gymnast "grabs" the spotter's hand). The gymnast can also squeeze with the feet, too, instead of the knees.