How to engage the hind leg - Walk trot walk
You must establish a good active, rhythmical walk first.
Throughout your transitions you must have your leg on at all times to encourage the hind leg to step through.
Even in a downward transition.
You must keep the activity and keep thinking forward.
As with other transitions you need to tighten your core, sit up, and think small but active steps.
Use the elasticity in your elbows and keep your weight in your stirrups with your leg on.
Your posture must be balanced and looking ahead to allow the horses shoulders to carry you into the transition.
In this video at times I am looking at the top of the horses head which brings my weight over their shoulders therefore restricting them.