I was taught a simple, quick, light backhand slap to the face. The instantaneous reaction is for the student to put their hands up to their face, and that breaks the psychological lock they have on the controls.
The scenario was a small turbulence bump, the new student reacting by “bracing” themselves on the yoke, pushing it forward, with the sudden nose down resulting in further “bracing” - and a viscous circle panicked lockup on the controls the result.
Also never had to use it, but was it part of my instructor training from a very experienced 60 year old instructor.