Glider instructors have lived this scenario for years:

The Schweizer just would not stop moving around behind the Cub. Left,
then right and always up. Push-then too low, ease off the stick,
then too high. The glider moves first to one side and then with a zoom
across to the other.

"I've got it. Look at how my wings are parallel with the tug.
Your airplane."

In the USA, the primary means of towing a glider is called aero-tow. A
slow speed, relatively high-powered tow plane is attached to the glider
with a 200-foot length of rope that meets regulatory strength
requirements. The glider then flies a formation take-off and climb to
a suitable release point. Even for experienced transitioning pilots,
this maneuver is nearly as difficult as landing. In those first
several attempts, students end up in an ever-increasing departure from
the center.

Like most pilots, one of my initial challenges in learning to fly
gliders was mastering the aero-tow. As a CFI, I see my own
difficulties reflected in current students as they fight the controls
seeking the dead-weight spot where the glider just follows behind the
tow-plane. The dead-weight spot is the point where the glider is
imparting no other force on the tow plane other than the weight of the
glider. Any displacement from this spot makes the tow more difficult
for both the pilot of the glider and the pilot of the tow plane. When
conducting flight tests, I see many problems from applicants during
their demonstration of the Areas of Operation relating to Aero-tow.
Since control of the glider while on tow is a requirement, it carries
some weight on the outcome of the flight test.

Nearly all two place sailplanes are constructed with a tandem
configuration. This adds some difficulty to training, especially with
the topic of transfer of control. A good preflight briefing settling
this is essential.

In conducting the preflight briefing, I follow the order of the flight
beginning with the take-off, through the various maneuvers, and ending
with the release from the tow plane. For the take-off, the dead weight
example holds. We balance the glider on the main wheel until airborne,
adjusting for any cross wind by crabbing so that no additional side
load is imposed on the tow plane during his take-off run. There is a
strong tendency for the glider to climb after lift-off as the tow
imparts the same force on the glider similar to running with a kite.
Were the glider to climb too high, the nose of the tow plane would be
driven into the ground causing at a minimum a longer than normal
take-off roll and a lecture from the tow pilot after your return.

Students will suffer tunnel vision when under stress. This becomes
evident while flying tow as the student becomes so fixated on the tow
plane and maintaining relative position that he will not see even the
horizon beyond. In order to maintain relative position behind the tow
plane, the controls of the glider must be operated not for coordinated
flight, but only to maintain position. Ailerons are used to match the
tow plane's bank. Any divergence from the tow plane bank will cause
lateral displacement, as the horizontal component of lift will rapidly
overpower the stabilizing force of the tug of the rope. Some
instructors will use the phrase in this case of "level your wings,
steer with your feet" to indicate that the wings of the glider are
not parallel to the tug. Too much bank in a turn relative to the tug
will result in the glider falling inside the turn and below the normal
tow position with a resulting slack towline, while too little bank in a
turn will result in the glider accelerating around the tug and climbing
above the normal tow position.

Relative height behind the tow plane is maintained with the elevator.
The typical training glider in the USA is the Schweizer 2-33. When the
2-33 is towed, it is flying at a speed that is nearly 40% above normal
design flying speed. This will cause a need for constant forward
pressure on the stick in order to prevent climbing well above the tow
plane. Lateral position is maintained using the rudder-not the
ailerons. At any one time, the controls in the glider are being
utilized to maintain position, not necessarily coordinated flight. The
goal is to merely be pulled up as dead weight.

Displacement from the normal position will eventually result in a slack
line and if the slack is not carefully removed a broken towrope will
result. It is for this reason that the PTS requires a demonstration of
slack line recovery. It goes without saying that any slack that
develops not be allowed to foul the glider, so move away from any loop
that develops. The action of moving away will tend to remove some of
the slack. At this point the tow plane is relieved of the dead weight
of the glider and will accelerate. Since the glider is not currently
being pulled by the towrope, it will decelerate. If no further action
is taken, and no other force applied that increases slack, such as
turbulence; the slack will just go away. Any attempts to force a
reduction in the slack by using an excess of drag will cause the rope
to snap taught and possibly break. The PTS standard for the performance
for aero-tow is only smooth and effective use of the controls.
Snapping the rope would not be very smooth.

During the typical glider flight, the tow is approximately 5-7 minutes.
This concentrated time can make students confident for the remainder
of the flight or distraught over earlier performance. Consider staying
on tow a little longer on later flights in order to give your students
more time. When I was a student, a long cross-country tow finally
solved my problems with the tow.