Half the group are laughing, thinking I didn't switch the rotary stop
on.
But I did, honest.

Anyway, I switch it to chisel mode, and chisel away at the wall. This
works well on the whole, but the chuck slowly rotates, so the flat
plane of the chisel does not stay where I want it to. It does not
rotate unprovoked, but only when a torque is applied, e.g. when
chiselling against a bump.

Is this meant to happen? Or is it broken? Or am I doing something
wrong? Or something else?

Thanks!
David P.