Lower leg in sitting trot?

Wbs are notorious for extremely bouncy movement due to their big paces. Of course some are easier than others but in general they are not easy to sit on.
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Sounds like he needs work. Not that you're doing anything wrong, you should be able to hold your leg against the horse and not have him mistake it as a cue. It will be good practice for you to ride him though and work with his gait. Then you will appreciate the warmblood :)