Your horse is expressing cadence when you appear to be moving in harmony with the horse with well marked regularity, impulsion and balance.
I like to think of cadence with a musical connotation.
Think of the metronome used to produce regular, metrical beats (clicks) and used by musicians to keep a steady tempo. It is used to work on issues of irregular timing. Your horses’ paces should be regular 4 beats for walk, 2 beats for trot and 3 beats for canter. Regularity of the paces being fundamental to dressage.
The first level of the German scales of training, rhythm and relaxation is the where we start to ensure we have cadence. Without relaxation you will not achieve the spring in the gaits that you need to show cadence.
Additionally, when a horse is not in balance (See B is for Balance) and transferring too much weight to the forehand, issues and flat gaits will result. Athleticism and energy are needed but the expressiveness you need comes from developing the engagement of the hind legs.
Here’s what to do. You will not go into the arena to work on cadence. You will work on rhythm, you will work on relaxation, you will work on balance and when you have achieved a certain level of each, your horse will be expressive and will show cadence in his paces.
How do you know if you have it? As ever … you will feel it!