Dressage riders, trainers, and judges often refer to "engagement".   What is this?   Hopefully, the two photos on this page help demonstrate and explain "engagement".
The horse shown is a Dutch Warmblood mare, Jeddien, born in June 1991.   She has progressed from UK Preliminary (USA Training Level) to Advanced Medium Level (USA 3rd / 4th Level) in dressage.
Moving from the lower levels to the medium levels involves demonstrating more engagement, something which is developed over time.
In the first photograph, we see Jeddien standing at rest.   This is the same stance she takes when standing in the fields, looking out in the distance, or when standing in her stable.   Jeddien's neck is long and relaxed and the majority of her weight is over her front legs.   She is at rest.
Take a good look at the hind end.   Notice the angles, the placement of the hip, the croup, the tail, the point of the buttock, the lower legs, and so on.   Check the height of the stifle relative to the elbow.
Now compare these aspects with the 2nd photo.   Where you see differences, you are seeing signs of engagement.
With dressage training focusing on both muscular development and emotional acceptance of the work, the horse learns to engage.   This means she learns to carry more weight on the hind legs.   To do this, she must learn to close the angles and "sit" behind.
To teach engagement, is to go through dressage training.   Transitions, shoulder in, and steps of rein-back are great tools for teaching engagement and developing the muscles as well as the mind of the dressage horse.   Take the time needed to do this and you will see results.