Where? From what I recall, Whis says UI makes the person able to attack/defend without having to think about it, a response faster than even subconscious reflexes. IIRC, Goku getting better at it was more about him "getting the knack of it" than UI instantly making him stronger... Similar to how he and Gohan spent all the time as Super Saiyan to get better at using that form.
Goku becomes stronger with the transformation because... Well.. That's what saiyan transformations do.