Fuel tank secondary for Pyro.
Upsides:
Fire has longer range and no damage fall off.
+100 ammo capacity
+25 health

Downsides:
The tank itself has 100 health. If the backpack is shot, Pyro takes no damage, but if destroyed, it blows up and kills the Pyro.
Flame particles are affected by gravity.
Less spread.

In terms of item description, it could be sumarized as:
Carries a lot of high pressure liquid fuel.
+25 health.