The Guild Of Wrench & Grenade Launcher - Engineers & Demoes, Creation & Demolition...

The Detonator is a secondary weapon for the Pyro. It is only available in the Team Fortress 2 Beta. Currently, it uses the Flare Gun model.
The weapon's alt-fire detonates the flare in mid-flight, creating a small explosion which ignites enemy players. The knock back from the explosion can also be used to Flare jump in a manner similar to rocket or sticky jumping. Players wielding the Detonator have a 20% explosion vulnerability. It was released in Team Fortress 2 Beta on February 11, 2011 Patch.
Since the item is still in an unreleased stage, these attributes may be changed or the weapon may be scrapped entirely.
- TF2 Official Wiki

I am quite impatient for this nice gun to appear in game!