As in any sport, muscle memory is a key component so that the desired movement becomes automatic and with little step-by-step thinking. This requires repetition. Consider any repeated motion, such as a simple forward roll from a squat position. Now do that 20 times. Consider running full tilt, jumping over a small object, landing and executing a forward roll. No do that 20 times. After a 60-90 minute class, you can expect some pleasantly sore muscles the next day.
It’s little different than running in a playground for hours on end, except there is structure in the movements learned and a philosophy that accompanies the discipline. One of Parkour’s founders, David Belle, has the attitude that parkour is a method of self-refinement, used for learning to control and focus oneself.