On casting Blood Flow you get a 15s buff that increases your melee damage slightly. Stacks 5 times. Blood Flow will refresh the buff duration, so you can pre-stack it before fights and then maintain it. Additional ranks increase magnitude of the damage buff.
Doesn't multiply combo damage only adds to it and also your usual "white damage".
EG If your Crush Armour would normally hit for 500 and you have this buf specced to gove you +20 2HB damage, then your Crushed Armour would hit for 520
Good feat to get at level 10-14 when Bear Shamans are especially weak but you will have to stack it 5 times before each fight which is irritating.
As you rise in level, there's no need to stack this buff as white damage becomes less significant.
At level80 stacked 5times = +43.2dmg