Each weapon has a damage type. Each damage type appears to have a 'proc effect' that can occur. There are instances of this not being listed in the weapons details (right click and examine) so please check carefully. If the weapon has the damage type and has +Effect Strength (which you want anyway), this will definately proc though, so its not too large a consideration. It looks to be a pretty low base chance to proc for me at any rate, something in the region of 1-2%. The effects are as follows.... Fire - Chance to Ignite (burn) - Target loses 5% of its total HP per second for 5(?) seconds.