I'm gonna be kinda blunt. And I'm a bit ashamed of myself.
There's a lot of posts on this. A lot. Really, a lot doesn't suffice. There's a ton. Sigh. You wouldn't even have to do a search, they're right on the first page for ya. It's quite glorious. In any case...
Both strength and skill play a part in backflips. I can't tell you how much strength it takes, as there's no number for that sort of thing. You need enough strength to where you can jump high enough to complete the rotation for your level of skill. In other words, it's based on you. If you want to learn correctly and more efficiently, find someone in person that can show you and lead you through the steps in real life. This is ultimately the best way to go about it.
