The simplistic "shift your weight to one side to turn" advice given to wuffos is certainly part of the turn process, but by no means the whole story.

In this tutorial, we'll consider the roles of pitch, wing loading, yaw and adverse yaw, and how all these things come together to make the perfect flex-wing turn.

The role of pitch in a basic turn

Consider if we were to just somehow roll the wing. It would immediately begin to slide slip toward the downwind wing. Because of the resulting drag on the swept inside leading edge, it would be retarded and the glider would rotate nose-down and toward the inside wing. We'd be turning, but it would be more of a fast descending spiral.

However, if the pilot were to push out the bar as the wing rolled, as long as airspeed is kept above stall, the glider will follow a flatter curved path.

The higher the bank angle, the more pushout is required.

In general, of course, this roll/pushout is performed in one smooth action. The diagram shows the actual body position and arm movement used in initiation a typical turn in a hang glider.
 

The mechanics of a turn in more detail

Most flex wings are swept, with nose angles between 120-140 degrees for modern designs. As well, the frame leading edges  and sail are flexible. This produces somewhat different aerodynamic effects compared with a normal fixed-wing aircraft.  The cross-bar of the wing frame is allowed to float slightly with respect to the keel, and this, along with some other geometric considerations allows the sail to "billow shift".  That is, if you were, for example, on the ground, to grab the trailing edge of the sail on one side, and push it up, you would notice the other side of the sail would become slightly flatter and tighter.

Let's put that together and see what really happens when a pilot speeds up a fraction and moves weight to one side of the control bar (lets' use the left side as an example)

The wing loading on the left wing increases compared to the right. The normal response of a wing to a higher wing loading is to speed up - so that's exactly what the left wing does compared with the right. So the glider exhibits a YAW TO THE RIGHT, which is not where we want to go.  This is called ADVERSE YAW, and is common to most aircraft.

In 3-axis aircraft, the downward deflection of the aileron on the outside of the turn generates much the same problem.  The solution in 3-axis machines is to include "differential ailerons" where the inboard aileron deflects up considerably more than the outward one goes down, plus liberal, (in some cases), application of rudder to counter the yawing tendency.

To make things worse for we flex-wing pilots, the existence of billow-shift can make adverse yaw worse.

The extra load on the inboard wing causes the sail to shift to that side. That tightens the sail on the outside wing, increasing its angle of attack, while decreasing the angle of attack on the inboard wing.

While this means that lift is increased on the outboard wing compared to the inboard, thus providing a faster roll, the increase in lift also incurs a drag penalty - so the outboard wing is more draggy and the glider will tend to yaw toward it.

The type of drag we are dealing with is "induced drag", and this is greater at low speeds. Thus, adverse yaw will be worse the slower you are flying.

Now we have our example pilot on the left hand side of his control bar.  His left wing has moved down somewhat (bank), and forward (adverse yaw). He is also slipping slightly to the left, as a result of the bank.

It is this slip which gets us back on the right track.  The glider is effectively "flying sideways" momentarily, and the drag builds up on the left hand side. Thus the left wing is pushed back, starting a strong yaw to the left.  The right wing begins to move faster, developing even more lift, and this accelerates the bank.  The pilot at this point begins to push out the bar, increasing the angle of attack of the whole wing and (hopefully) balancing the turn; returning to centre when the desired bank angle is reached.

This of course happens in a continuous flow, not in steps.  The roll and slip begins almost immediately the adverse yaw occurs.  Whether the pilot actually observes the adverse yaw occurring is largely dependent on the glider design and the speed at which the turn is initiated.  In slow, floater gliders, it may well be marked, whilst in fast trike wings, it will be negligible at most normal flying speed.

Nevertheless, hang glider pilots can use yaw to make turns more efficient.  For example, in a slow speed left turn, a pilot could quickly swivel their body right  (producing a slight left hand yaw), following with movement to the left hand side of the bar as normal. This effectively cancels out the adverse yaw component and will get the glider into the turn faster and flatter. This isn't an option for trikes with their hang points restricted to 2 degrees of freedom.