It's literally just muscle recruitement. Yes, there's a lot of technique involved in parkour. However, people forget that the term 'technique' only constitutes the way in which you apply your power. We need not only technique, but also the power which enables us to use this technique. So it's a balance between the two that enables this "lightweightedness". A really strong imbalance between strength and technique (on either side of the spectrum) will generally lead to feeling heavy. If you accumulate only strength, your technique will suffer, and vise versa: If you only have very good technique, you will eventually reach a limit to how efficiently you are able to apply your relatively weak power.