A mechanical barograph is an analog instanteously recording device.
Thus a quick 200ft notch is sure to be registered.

Howerver with electronic devices, you might not get a clear recorded
event by performing a quick 200ft notch, as the recorder only records
once every XX number of seconds (XX depends on the recorder and the
settings). Bascially the recorder could miss the notch altogether.

You could set your recorder to take a measurement down to 1 second to
ensure capturing that notch, however this means that the entire flight
will be recorded at this higher resolution. This is not necessarily a
bad thing if the recorder has enough memory to store enough captures
for a long multi-hour flight. The new EW Microrecorder can record
down to 1 second intervals and can capture 30 hours at that rate. The
Cambridge 302 can record for 25 hours at 1 second intervals. However,
the Volkslogger is only good for 2.5 hours at 1 second intervals.

I think that you have two choices.

* Notch - I believe that all FAI loggers have some capability to
record "pilot events". These are recorded at the highest rate
possible for that device. This feature is typically used when you
enter a turnpoint to guarantee that you have taken a reading but can
be used for capturing the notch. Reading from the Volkslogger manual
"Activates (default) or deactivates the acoustical signaling of the
fast recording (1 second interval, Fast Fix). The fast fix mode is
triggered by pressing Key (3)," Key three is bottom right. Click
just before the notch and then just afterwards.

* Circling - I don't know exactly what the rules are on this but I
believe that performing a tight circle (which cannot be done on tow)
verifies that you are off tow. I do this.

Of course during contests this is a mute point as the "start" is
marked by leaving the invisible start cyclinder or line, not by the
other methods shown above.

Good Luck, John