Really? I picked it up somewhere in my instruction. Something to the
effect of "Think of the yaw string as an arrow with the head attached to
the canopy. It points to the the rudder pedal you need to push to
coordinate." Seemed very natural and simple to me.

Yes. I use this most often with transitioning power pilots who are
taught to "step on the ball" of the inclinometer ("step on the head of
the arrow"). With ab initio glider pilots I start with telling them to
add rudder on the opposite side of where the tail of the string is
pointing. If that doesn't work, I switch to the above.

Tony V.