Base wanted Tess to know she enjoys the feeling of 'melding' together - riding from her body, not her mind. Base said she was 'talking to her body' and called it The Dance.
The amount of sensory input it takes to balance ourselves on a horse is not something our conscious mind was designed for. We couldn't even begin to narrate the complex symphony of information our body is basing it's adjustments on. Our unconscious has the processing capacity to respond with dexterity and nuance. This 'body to body' communication is what Base enjoys.
The centaur-like image she showed made it difficult to discern where one body ended and the other began. This 'melding' extended to intention. Pace and direction was a shared conversation that felt like play.