How do you do loads backflips in a row without stopping

No it doesn't ^
It depends on your position and the gravity center at that point (position).
If all these are ok then you can do a backflip by building up momentum by swinging your arms from down to up with enough speed and strength to get you to do a little of a jump without using your legs, when you get that, add even more momentum by using your legs I usually relax the knees and extend them 10 frames before the jumping time then contract them 3 or 4 frames later(you'll notice if you have enough momentum or not).
Now the movement of your arms after jumping also affects on the jump itself, allow a period of 10 or more frames after the jump then lower your shoulders and extend your pecs, then after being at 180 degrees to the surface, relax your arms (that includes pecs)/ or contract the pecs and raise the shoulders.
Remember to relax the knees all the way through, tho.