Honestly you should ask your instructor (and if you don't have one, get one for sure) to put you on the lungeline and help you figure it out.
It's an upward motion of the hips, not a forward back motion. Same as trotting, an upward motion. Where most people mess up is they push with their hips which causes their butt to get some air and slam back down on the horse or lose its timing. Then you aren't with your horse at all and it's hard to get back to that.
Hence lungeline work.
I kept this reply in mind today while riding and it helped me so much! Thank you.