Change fate allows you to automatically get a roll of '20' on one saving throw or ability check (the next saving throw or ability check you make after manifesting the power). You can also choose to get a roll of 20 on a skill check (both provided the skill can be used untrained). You choose to manifest this power instead of rolling the d20. You can manifest this power instantly so that you can gain its benefits in a clutch situation. Manifesting the power is an immediate action, like manifesting a quickened power, and it counts toward the normal limit of one quickened power per round. You can use this power even when it's not your turn.
Your effective roll of 20 guarantees success on a saving throw. Your effective roll of 20 may or may not give you success on an ability check. If you use the power to make a skill check, apply only the roll of 20, plus your relevant ability modifier—ranks and other modifiers do not apply.