Some horses are naturally blessed with impulsion, nevertheless, with correct dressage training, horses that are not so talented in this respect can be improved.
In this video with Uta Gräf and Deliano, you will see how the the expression can be improved with appropriate training.
You will see that promoting impulsion starts in the warm-up phase. The horse should be encouraged to move actively whilst keeping a soft contact with the bit. The transitions are helpful for improving self-carriage as well as responsiveness to the riding aids.