JeffG
Contributor
Upwards would hint possibly at your trim being wrong - if slightly head down during it the motion can pull you upwards.
Style of fins seem to make a huge difference to the ease of performing a backward kick.
Actually, I would bet money that the OP is rushing the back kick.
Most new back kicking divers hold the position of fins close to their butts to long and don't hold the fully extended position long enough.
With the fins close to the butt, the diver starts moving in the head down position. They don't hold the fully extended position long enough to straighten out before doing the next cycle.