Jack wrote
I believe scenarios where the
student and the instructor should be on the stick at the same time are rare.

Rare is not the same as non-existent.

Either the circumstances permit the student to control the aircraft or they
do not.

The problem is that we want the student to get to where they do as
quickly as possible. Sometimes this is best done by occasionally
'helping' the student, so that it doesn't become necessary for the
instructor to take full control.

The Instructor may certainly "guard" the stick (e.g., place a
protective ring around it with his hands) in order to prevent gross
over-control in circumstances such as aerobatic/formation flight when the
student is not yet proficient or the Instructor is unsure of the student's
level of proficiency.

Guarding the stick is a completely different matter. The nice thing
about a tandem trainer is this - the student doesn't know you're
guarding the stick because he can neither see you nor feel you on the
controls.

the student can
expect intervention when necessary but should continue to fly the aircraft
to the best of his ability until commanded otherwise.

There are few things more annoying than feeling someone else's unbidden
control inputs. An instructor who "rides" the controls is not yet ready to
instruct.

I think this is exactly right. If you need your hands/feet on the
controls to know what's going on, then you need more experience flying
before you try to teach someone else. That doesn't change the fact
that you sometimes have to help a student.

Usually the help is necessary on tow. Ab initio students may be a
different matter (I've never trained one) but it is my opinion that a
transition pilot should be flying on tow from his first lesson onward,
and that's how I teach. Practically speaking, that means I give him
the plane at about 500-600 ft AGL (the altitude from which a return to
the airport and normal landing into the wind can be accomplished
without drama if he really hoses it up) and he starts screwing up.

Nothing I do will change the fact that it takes time to learn to fly
on tow - stick time. If my hand is on the stick and my feet are on
the rudders, my student is not learning. At this point he is making
mistakes faster than I can talk, so unless he is doing something
systematically wrong, anything I say is useless - he just has to get
the feel of it. Demos are all well and good, but I have already
showed him how to fly on tow, and let him feel it (by following me on
the controls) by the time we've made it to 500 ft. Now he has to do
it, and the more time I give him on the controls, the quicker he will
learn.

Since he will not be able to control the sailplane on tow for more
than a few seconds at a time at first, I basically have two choices.
I can let him screw it up until I have to take control, then recenter
on tow and have him try again. Or I can slap the stick or tap a
rudder, knock him back into the game, and let him keep struggling.
The latter option gives him way more stick time on tow, and gives him
a fighting chance of having the tow thing figured by the second
flight.

The general case is this - when it is clear that the student
understands the basic concept, and we're just waiting for the hands
and feet to catch up with the brain, then a little help can be useful
if that's what it takes to let the student keep practicing safely.

I really can't imagine a situation where it would be appropriate for
the instructor to 'help' a student on the controls at altitude and not
on tow.

If CFIGs out there feel that gliders present a special case, I invite your
further clarification.

I certainly don't.

Michael