A flick is a action designed to take advantage of the foils bendiness and flexibility in order to score on some hard to reach place (such as the back).
You will need some kind of target for this (perhaps enlist a partner). Your first need to check your distance, and you may need to adjust it. The ideal distance is about slightly closer than extend distance.
Performing the Flick:
Get in your en garde and face the target, get within the slightly more than extend distance I mentioned earlier. Extend your arm up, and forward, bending your wrist slightly. Allowing the foil tip to drop slightly, and then (using pretty much only your fingers) you suddenly stop it. Causing the tip to bend and hit. You sort of let the foil fall through your fingers, and then stop it.. Knowing when to stop, and what distance to begin at are paramount to performing the flick, so play around, or ask a coach. A important note: your hand/wrist need to always be higher than the tip when flicking, otherwise it will not work. Squeeze your thumb down at the same time when you bring the foil down this will help the tip stick, and press down longer (ensuring you actually get the touch).
Something to keep in mind:
The flick is a very difficult and advanced thing to do, (you probably already know that). It requires a lot of practice to be able to perform the technique properly, and time in this case is your friend. So practice it a lot, until you are confident enough to use it in a bout.
I hope you know have a good understanding of how to flick, and happy fencing.
Until next time,
-Fuzzy