How to engage the hind leg - Walk halt walk
As simple as it sounds you do need to practice this as it is the foundation of transitions.
You need to have an active walk, not too long and free, and it must be in a rhythm.
If you are not sure count 188.8.131.52 and keep repeating making sure the counts are even.
The activity needs to come from behind. If your horse loses activity refresh the hind leg.
When you ask for halt, 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.
The horse will then step up into the halt.
Always correct the halt if not square.
My first halt is uncorrected which demonstrates the horse moving off with the fore leg.
The other halts are corrected and you can see the horse stepping under from behind.
Ensure your move off is positive with clear instruction to your horse.