Adverse Yaw

Actually, if the string is in the middle, you are in a skid (unless the
string is attached at the CG of the glider, which usually makes it a
bit hard to see). Ball in the middle would be correct - but few US
gliders have slip balls (may be more common elsewhere?).

See Dick Johnson's explanation in a recent Soaring magazine article -
or Moffat in his first book.

A little slip helps reduce the aileron needed to counteract the
overbanking tendency, and often inproves climb rate. I know it does in
my LS6.

I doubt a little skid hurts much, and the difference is probably pretty
small - better just to bank steeper and slower!

Kirk