Celerity

Celerity rules are on page 142 of V20 Core.

On Modern Nights specifically, using the first dot of Celerity does not actually constitute a Masquerade Breach: you're being remarkably fast, but not inhumanly so. Levels 2+ of Celerity are, on the other hand, Masquerade Breaches. That doesn't mean that you're off the hook if you pair that extra action with something that is itself a masquerade breach.

As stated in the v20 core, your extra actions from celerity must remain physical actions. Celerity speeds you up, but it doesn't speed up the world and machines around you. A car will only drive as fast as its engine permits and so on, so you cannot use Celerity to make a machine operate faster or someone else's brain comprehend commands faster. So, you cannot use the ranged combat moves 'Automatic Fire', 'Strafing', and 'Three Round Bursts' more than once in a combat round (but you can use all the other ones) because these are semi-automatic and automatic weapon moves that rely on the speed of the actual gun. Social disciplines like Dominate are ineffective because the person you're targeting it on isn't going to be able to comprehend it at that speed.

Beyond that, use of Celerity works exactly as stated in the v20 core.