Jim,

There was a thread on this subject a year or two ago, you might want
to search the archives.

The simple aerodynamic answer goes like this: The circle is traced by
the glider's center of gravity, which means that your nose extends
beyond the edge of the circle. To visualize, draw a circle, then draw
a line tangent to it. You can see that the nose and tail of your
fuselage transcribe larger circles. A yaw string forward of the cg
will show a slight slip for a coordinated turn (that is, coordinated
at the cg, or wing). If your yaw string is straight, then you are, in
fact, slightly skidding the turn. This effect also exists at the tail,
requiring you to hold a little bit of rudder into the turn (but not so
much that you straighten out the yaw string). Obviously, the longer
the arm, the greater the effect.

I've tried to observe the difference between the front and rear yaw
strings on a G103, but the canopy edge generates too much turbulence
to mark any clear difference.

For practical purposes, the slip is small (5 to 10 degrees).