It's a throw back to the days when we worried about things like "How long
will GRP gliders last?". The idea is to check to see if there are any
structural changes happening in the wing or spar. It is checked by
assembling the glider, and "bouncing" the tips to the frequency where both
tips rise and fall together. Note the number of oscillations per minute on
your watch. Make a note in the logbook. Check it next year. It should
never change significantly (and it won't). I had an ASW-17 for 18
years. It never changed. Maybe this is just a Schleicher thing.

Roy B.