Both the string gasket and O-ring gasket Fire Pistons work well however the string ones are a little more persnickety.  Lubricate the string (or O-ring for that matter) each time you attempt to get a coal with Vaseline (included) to help the seal the cylinder better to provide more "compression of the air".  The more compression, the higher the air pressure inside the cylinder, the easier it will be for the heated air to ignite the tinder at the end of the plunger.

Using Char cloth as a tinder seems to ignite (and form a glowing coal) a little easier than the tinder fungus, however both will work to get a coal.

NEVER drop the tinder into the cylinder as Les Stroud did on his television show "Alaska" episode.  This will not work (unless you have a good film editor that uses "camera magic". 

Dropping tinder (or leaving tinder) in the cylinder presents the potential problem of having too much tinder in the cylinder so that the piston "bottoms out" in the cylinder as it hits the excess tinder.  This  will crack the end of the piston cup.  In addition, it will had to get the coal (if you even get one) out of the cylinder

Here are some tips as to how to "correctly" use your Fire Piston...

Place the plunger as far out of the cylinder as you can (that means push the plunger down into the cylinder until the cylinder just covers the gasket).  This will provide the longest "stroke" and traps the most air to be compressed.  More air to compress means higher pressures, which mean higher temperatures in the cylinder, which means a successful lighting lighting coal.

You need to hit the plunger in HARD, FAST and SQUARELY and then IMMEDIATELY pull it out and blow on the tinder.  When I say hard and fast that's what I mean, HARD and FAST.  I say SQUARELY because an "off-center" hit can put sideward pressure on the plunger and crack it.

Try using the char cloth at first (instead of the tinder fungus) until you get the hang of doing it.  Also if your using the tinder fungus scrape a little of the top of the fungus with your fingernail to "fuzz it up" to make it easier for the heat to ignite a coal.

If you try in the first time and no coal forms IMMEDIATELY try it again.  The first time may have heated up the tinder to "just below" the ignition temperature and a follow up "hit" will ignite it.  Remember the air has to heat the tinder to over 700^oF to begin glowing as a coal!

Humidity also makes it a little harder to get a coal so on damp days you may have to use 2-3 hits to get a coal to form.