Beginning To Ride On Contact
I do not ride on contact for quite some time when I am first starting a horse under saddle. My program is all about stepping stones that build upon on other so you never need to go back and reteach a horse anything. If you begin riding a green horse on contact, or even asking for a stop with two reins too early you have set your horse up to fail. By asking too early, the horse does not yet understand how to soften and may brace against you, raise the head and subsequently hollow the back and disengage the hind end by letting it trail behind. Your job as a rider becomes more difficult then as you must reteach your horse to respond to something they have already learned.
If you haven't already, I recommend looking at this previous article of mine. It covers some of the prerequisites a horse should know before you pick up on contact for the first time: https://caitlynhppr.wixsite.com/chhorsetrainingtips/single-post/2017/04/28/How-To-Get-A-Better-Stop-And-Soften-Your-Hard-Mouthed-Hot-Strong-Horse
If your horse is not soft go each rein independently, can not back up softly and properly, and stop on their hind end, it is very unlikely that the horse will respond appropriately when you pick up on two reins. In addition to this, having some basic body control helps out immensely. A horse should be able to move off of your inside leg and outside leg.
At this point, introducing contact becomes a walk in the park. You know that if you pick up on both reins, your horse will soften the jaw, relax the poll forward, use the muscles along the top of the neck, shift their weight to the hindquarters use their back properly and raise the shoulders so they can stop properly. All you need to do, is then add leg, cluck and encourage your horse up into the bridle. When I first introduce contact, I do so at a trot. A trot has enough forward momentum to help you out. Starting at a walk, it can be easy for the horse to stop before you push them forward into contact. At a canter/ lope, a horse may try to rush forward when you add leg out of confusion.
As this is a new behavior, it's important to release for the try. After one, soft stride release and go back to a loose rein. After a few moments, ask again. As your horse learns, you can ask for two strides at once, then theee, four etc.
If at any point you feel your horse begin to lean on you, you have plenty of tools to fix this. The first thing I recommend is to go back to a transition to the back up. This will soften your horse and rebalance him.
Another thing you can do is ask for contact on a circle, serpentine etc. bending your horse will encourage the inside hind to further step forward and help your horse not to rush forward.
Understanding good vs poor contact
Lets compare the above photos of me riding two separate mares.
In the first, the palomino mare is using herself well. From her hindquarters she is stepping well under herself, when the right hind lands, it will be in the spot the right front left. This lets us know that the hind end is working well. A horse that isn't using himself well will have an asmetrical action from front to back. You may also find that when slowed down, it is not a true diagonal trot. Moving forward, she is working through her top line, her back is engaged and the top side of her neck is working. The muscles on the underside of her neck are relaxed which is wonderful. Structurally, her poll has remained the highest point of her neck. At this position, the poll can relax, letting the head fall into position while her jaw softens to my requests.
I on the other hand am hindering her. I have a bad habit of leaning forward which makes it more difficult for a horse to balance properly. However, I have kept my leg underneath myself, allowing my leg to cue her correctly. My hands are maintaining a nice contact with a straight line through the reins and my arm.
In the second photo, this grey mare is faking it. This is a screenshot of a video of my initial evaluation of her once she came to my barn. After riding her saddled, I hopped on bareback. This mare displays something commonly seen with western horses. She has been ridden front to back with emphasis on head position, she has learned where to set her head but the rest of her body isn't engaged.
Lets dissect her. From the hindquarters you can see that she does not have a very forward trot, she is short strided. Without much energy to work with, it is difficult to ask a horse to balance themselves differently. This mare is not working over her topline well. Looking at her neck you can see how she is curling behind the vertical. When a horse does this, they use the muscles on the underside of the neck to pull the nose backward. This is very typical of a horse that has been bumped into position with the reins. When a horse gets behind the bit, the entire body suffers. Instead of everything working together, her neck is working separately from the rest of her body, making it impossible to use herself correctly.
Being bareback has put me in a chair seat. While my torso and hips are able to sit and drive her forward, I am not as effective with my legs. You can see the difference in pressure between the two mares. On the palomino, a soft squeeze of the calf kept a nice, forward trot. On this mare I am asking with my calf and heel go even maintain a shuffle.
How to fix this
With a horse that hides behind contact, your work becomes more difficult. Although it can be tempting to keep a softer hand, don't. Too loose of a contact lets the reins move around which furthers the cycle of a horse curling back because of the inconsist movement on the bit.
Keep a soft but steady contact and add more leg. Doing this on a straight line isn't the most beneficial. Many horses will keep their head curled in and just rush foreward on the forehand. This is where bending becomes your friend. You want to constantly be bending your horse and using your inside leg to really encourage the inside hind to step up and under. You can even use a dressage whip to back up your leg.
These types of horses should be treated like they are just learning. If you feel yourself raise the poll and come into the vertical or slightly in front while the hind end works better, release! Wait a few strides then ask again.
When your horse begins to work for multiple strides, keep it different. Bend one direction for a few strides, then the other. Changing direction will help as your horse will need to travel correctly in order to constantly be moving the shoulders either direction.
Remember, your horse is still learning and building muscle. Give your horse frequent breaks and do not ask for too much at one time.