The BGA signals were developed long ago, before modern gliders and water
ballast. They don't depend on the glider attitude or control movements.
They were initially designed for winch launching.

The way I see it the wing runner/signaller is in a better position to see
things that could go wrong, such as cable overruns or people landing, than
the pilot.

Chris

At 17:40 30 March 2018, Tony wrote:
The signals were developed in the late 1940's, before water ballasted
gliders (besides the SGS 1-21) and back when gliders mostly sat on their
noses with the tail up in the air for all to see.

The SSF strongly recommends that when the glider and towplane are radio
equipped (as almost all are these days) that radio calls be made along
with
standard soaring signals.

I generally don't let the ground crew hook up the rope until I'm ready to
launch and I definitely don't give them a thumbs up unless I'm ready to
launch.