Our regular glider DPE asks me to fan my rudder at 1000 feet AGL
during a checkride to see if the glider pilot candidate understands
what the signal means. We do it at 1000 feet close in to the field so
that if he mistakenly releases, he can make a normal pattern and
land. Our instructors are good at ensuring glider students get to see
the rudder wag and the waveoff during their training.

Radios are primary for communicating between the glider and towplane
but the battery in the glider is subject to petering out so the visual
signals are there as a backup.