When you do a forward you pull the sail in with your backhand, when you do a grubby you push the sail out with your backhand.

Yep, it's the backhand. To fix it try them further off the wind to start with. Make sure you turn downwind and then jump without delay... ie down't keep sailing downwind straight looking for a ramp, use the momentum of the turn to give you rotation. Keep both arms bent as you turn downwind, then when you jump it's easier to think about just pushing out with your back hand but make sure you keep your front arm in. Keep your weight centered over the board and as far forward as possible. Good luck, Grubbys are fun... Now if you can Flaka please give me the key... it's proving the hardest move to learn ever!