I know I started this series a while back but I wanted to get back to it. I talked about Rhythm previously and posted a video example.
Often with rhythm steadiness and consistency, Your horse will naturally start to relax. He will lower his/her head set a little or a lot depending on how soft they are naturally. I work on Rhythm with a completely loose rein first (if you don't trust your horse yet here, don't do this) You can work on a medium loose rein if you are more comfortable.
When I feel relaxation and consistency melting together and my horse starts to relax his neck position down, I know not only are muscles more warmed up but, the mind and body is relaxed too. A horse cannot lower his head naturally until he his in a calm relaxed state. This can be done with a bit, without one, in a sidepull or rope halter. I can do it in any of those. If you have a hot horse (quick rhythmed horse), or an unbalanced horse (this can manifest as quickness to) you can use serpentines and spiraling circles to relax the rhythm down and or help him balance himself up. Bends and circles naturally cause a horse to balance without you doing much. If you have a more sluggish horse, stick to straighter lines and larger figures. Relaxation may not be as much of a problem in this type but consistent rhythm might. He may not want to keep the trot or canter or walk for that matter. This can cause the back to be tight and body to lose relaxation due to lack of impulsion and connection. I usually work with all these things at Liberty and on line first to introduce the concept before riding.
Once I feel that my horse is warmed up and relaxed, I may go to a shorter but still slightly slack rein. I will then repeat the Rhythm and relaxtion work to get the same feel on the working contact. Every horse is different so listen to him when he is communicating things and ride with a feel for the individual not the exercise.