ED drugs do both, erections and blood flow. Use of them for ED recovery is to keep fresh blood infused into the abdomen, penis and the help the nerves and tissue heal or regrow. The warning is correct, as far as it goes- they will not produce erections without stimulation of some kind. They aren't gonna give you one whether you want it or not, ie: spanish fly effect. It only helps if you are willing to get involved...
Simple version: Erections are produced in the brain, by nerve impulses, with nitric oxide being released to trigger increased blood flow in the penis to create an erections. ED drugs facilitate this nitric oxide absorption, allowing greater amounts to do it's thing. One of the common blends of meds is the ED drugs taken with l Arginine- an over the counter that promotes nitric oxide release, hence enhancing the action of the pill. I have been taking 2000mg of it with my generic Viagra just for the effect it is supposed to give. Supposedly the routine use of ED drugs will help establish nocturnal erections, hence helping with recovery, infusion and regaining ability.