TOW PLANE Accident

...and another question: why do so many YouTube videos show the
glider pilot on tow NOT holding the release knob?
In fact it's hard to find a video where the glider pilot does hold the
release throughout the tow!
In a flapped machine that might require a flap change during the
initial stages of the launch it's understandable, perhaps for a few
seconds, but even after the flap change no one ever seems to hold
the release knob.
In the dim and distant past when I was taught aerotows they was
certain to be very loud angry noises from the rear seat if one let go
of the yellow knob. Has something changed in the instructor
manual?
Dave Walsh