BEGINNING THE HALF PASS
Remember, your position and use of body weight is very important when executing the half-pass. When your horse is established in shoulder-in you can progress to travers and half-pass. These are more collecting exercises and so must only be started when your horse is strong enough and the basics are well established to move on, not just because he has reached a certain age.
In the shoulder-in, it is only your horse’s inside hind leg that steps further underneath him, therefore, becoming more supple in the outside shoulder. With travers and half-pass both hind legs are stepping more underneath him and so both shoulders become more supple. For this reason it is more of a collecting exercise and so more difficult for your horse.
When riding a trot half-pass to the left, for example, (remember it is easier to start a half-pass from the centre line to the track than out of a corner), the left rein is the flexing rein and can also guide.
It is a common mistake for riders to hang on to the inside rein, which blocks the movement and flow of energy through the horse’s body. You should be able to see the left eye and nostril when your horse is flexed in this direction. Your inside leg is the forward driving leg and stays in normal position. Your outside rein is restraining and supporting and is the rein that stops your horse from flexing or bending too much in the neck. Your outside leg must move a little further back and is asking for sideways. Be careful not to grip with your knee or swing your lower leg too far back. Move your whole leg from hip to heel back a little.
As you start the half-pass, step a little more onto your left stirrup as this is the direction you are moving towards. Your right seatbone will come a bit more to the middle of the saddle as you bring your right leg back and your outside shoulder position back too. The trot rhythm should not change when you ride into a half-pass. If you lose forward tendency, abort the exercise and ride straight! Once you have the forwardness again you can carry on.
When starting your horse with half-pass, do not ask for too much bend and allow the hindquarters to trail a little, this will make it easier as is less collecting and will, therefore, be easier to keep forwardness.
Read more at http://www.horseandhound.co.uk/blog/shaun-mandys-dressage-blog-moving-home-half-pass-tips-476395#Fxr9aTk36Fl81eed.99