I used to fly sailplanes about 25 years ago and had never heard of Crow, so I had to look it up.
Crow braking: also called 'Butterfly braking', this rc glider wing function is used when landing. When Crow is activated the flaps drop down and both ailerons deflect upwards, all at the same time. This gives a very effective combination of drag from the flaps and reduced lift from the up ailerons, now acting as spoilers. The result is that a fast-flying glider can be slowed right down and, when landed well, can be brought in with very little forward airspeed.
It's common for crow braking to be operated by the throttle stick of the TX, although having it on a switch or slider is perfectly acceptable.