The trot is a diagonal stride that has a moment of suspension. In its purest, the legs work exactly together, and the horse should leave the ground in between strides. What I see as most bouncy is due to stiff joints, and/or merely big movements, which a rider must learn to ride. I have a little horse you can't even post to, she's so smooth: it's because she only moves her legs without flexing anything else. I think she's stiff; but my vet says he feels that's just the way she's made.