Energy flows through the horse like water through a hose. The horse’s energy originates in his hindquarters and travels forward through a relaxed back and topline. Maintaining a consistent connection via the reins is like adding a nozzle to your hose. Energy and relaxation were confirmed as part of losgelassenheit, and with the establishment of anlehnung the rider gains a tool allowing the horse’s energy to be channeled more precisely. By adjusting the hose nozzle, you can vary the spray from a light mist to a high pressure stream. By adjusting the rein contact you can channel your horse’s energy into schwung.
Speaking of expression, among the translations for schwung offered at an online translator were momentum, panache, drive, spark, sway, swing, impetus, buoyancy, verve, bounce, spunk, oomph, pep, zip, brio, zap, pizzazz, élan, oomf, and vim. 
Aren’t those more fun than impulsion?
Contact with the reins also gives the horse confidence to release even more energy from his hindquarters. It is for this reason that racehorses run “into contact.” So now riders have at their disposal more energy and more control! We can begin to influence how much of the energy goes forward, and how much goes up, increasing impulsion, suspension and expression. Now we can really begin to shape the horse gymnastically, developing his strength and beauty.  
<Young horses presented at prestigious <European sales have schwung as seen in their <expressive, impulsive gaits.
For Horses
For Riders

Riders not used to the power of gaits with schwung are often a bit intimidated at this point in a horse’s training, but it is a vital step. Moving slowly is more comfortable for the rider but requires great strength on the part of the horse, if it is to be done in good balance. Working on schwung is a key part of developing that strength.  
Schwung is usually translated as "impulsion." I think we can do better than that, but first let's understand what it means: