I was always under the assumption that ravage poisons reduce an enemies' max health, stamina or magika by a certain amount. Then I made a potion that has the ravage stamina effect and tried it on myself. It's description was "Concentrated poison damages stamina by 24 points" When i took the potion it was actually draining my stamina by 24pts for 10 seconds! I had no idea this would have a damage over time effect! I checked the active effects while the poison was active to confirm this. If you want to try this yourself, just combine a bee and honeycomb - its a potion that restores stamina and deals concentrated stamina dmg(unless you have the purity perk in alchemy)