Chicken
It is rude to reply to other people’s mail, but reversible controls are ones you can move normally during the walk around. Irreversible controls are those that are fully power operated, which appear locked solid during walk around.
For what you are discussing (spin recovery technique) reversible controls allow any aerodynamic loads on them to be felt at the pole. Indeed, as I am sure you brief your students, it may be necessary to apply considerable force in the cockpit to stabilise (reversible) controls that want to blow about which way. To a pilot used to flying only with respect to forces, having the stick or rudder thrashing about of its own accord can be quite off putting. This is when it is useful to encourage them to look down at the controls and physically put them in to whatever position is appropriate until more normal circumstances prevail.
While I think it is open to some doubt whether the precise position you are briefing them to move the stick to is absolutely correct in all circumstances, the notion of getting them to look at the stick and moving it by numbers in spin recovery is very important. Make them use two hands too!
I was always taught to apply opposite rudder (and really force same against the stop), then after a pause (of about half to one turn – which is just long enough to shout out loud PAUSE) PLACE the stick in the centre position (controls neutral when on the ground) and from there on move it progressively forward (not by feel but by position) until recovery starts (this does mean that if recovery does not start you will eventually be holding fully forward stick). As soon as recovery does start, really force the controls to central (especially the rudder, about which there must be little delay) and then execute a normal dive recovery.
This has worked for me since 1955 and since I have just about quit now all looks set fair for a 100% record.
Cheers