For every enemy you kill, there is a chance that your damage will increase. That chance is high, but not 100% - I killed six flies and got four damage increases. I'm still working on the exact chance of a damage increase.

This damage increase is not permanent, but lasts longer than for just the current room - it appears it goes down every time you enter a different room.

For every enemy you kill, there is a chance that your damage will increase. That chance is high, but not 100% - I killed six flies and got four damage increases. It's also possible that each kill actually increases your damage, but the value that it is increased is not always enough to display another bar on the damage stat.

The damage increase is only in effect for the current room, though it is possible for the game to display an incorrectly-increased damage value on rooms after leaving the room.