Some real dumb stuff being said in this thread.
Gainers are much more difficult to learn over a regular backflip. And running start to gainer? What the fuck? Telling someone to take a running start to gainer off a cliff is gonna break someones skull. There is no reason to take a running start to gaine.
OP, stand back facing the water, slowly move out over the water and stand on the dam with only the balls of your feet, so the rest of your foot is hanging over the edge.
you can jump down a few times backwards to get comfortable with the feeling but if you want to go straight into a backflip heres what you need to do.
(theres 2 kinds of backflips- Push backflips and falling backflips, falling is easier to get first so ill explain that.)
Fall backwards while looking up at the sky, act like you have a weight at the top of your head, bend back and make sure the last thing to leave the cliff is your feet, like they will whip up following your fall. Once your head is back and youve leaned back you will be able to spot the water/your landing. Using your abdomen bring the rest of your body around and land feet first in to the water. No tuck needed and youll do a full layed out backflip