UI isn't a transformation, anyone can use it supposedly. Its just ultimate understanding, perfected movement. Every movement you make is truly perfect as though you can see through each strike, each move you make is the least amount of movement with nothing wasted, each strike you take is the optimal path.
Absolutely perfected instinct, the body will automatically move in the most perfect way at all times. He doesn't need to think anymore at that point, the body understands the perfect form.
Goku during the Universe tournament still hadn't mastered it quite yet, as it still took a ton of stamina to be able to maintain it for very long, to have his body move at light speed to dodge every hit without even thinking about it.