Ultra Instinct -Sign- is the result of Goku being pushed to his limits and breaking past them, unleashing the deeper potential within himself. As shown, any time Goku breaks another one of his limits, he gains temporary access to this form.
Usage and Power
Unlike Super Saiyan God and Blue, this form doesn't utilize godly ki, as it could be felt.
Power-wise, the transformation transcends all forms of Super Saiyan God, as Super Saiyan Blue Goku amplified by Kaio-ken x20 was effortlessly defeated, and the Spirit Bomb that he used against Jiren contained much more power than Goku had himself, which Jiren also effortlessly defeated. After transforming, Goku could fight on par with Jiren, who was likened to a God of Destruction, and was suggested to be beyond that level.
The main detail of this transformation is that it provides the user with Ultra Instinct, the ability to have every part of their body respond on their own, without providing thought. This allows the user to engage in combat while the body moves and reacts on its own. The form is currently incomplete however, as Goku can subconsciously evade, but not physically strike, as he still needs to think out his method of attack, which limits the power of his blows. In addition, Goku can currently only use it in response to being pushed to his limits and it is a temporary form that strains his body significantly.
Note: Naturally, as Goku has been seen to achieve the form only by pushing past his current limits, each subsequent transformation into the form is much more powerful and efficient than the last.