It's that state you enter into when you’re so “in the zone” you don’t feel time passing.
There’s a fluidity between your body and your mind, and you’re totally absorbed by what you’re doing, so deeply focused on it that you’re hardly even aware of distractions.
It can feel as if time has slowed down, and like your senses are heightened. Your brain and body are so in sync that what you’re doing feels effortless.