Use of disciplines in combat is something that usually needs to be determined by common-sense and negotiation between players. But here are simple rules that will resolve most situations. Apart from powers that are always on (e.g. Potence, Fortitude), and powers specifically for use in combat (those with the Combat power modifier), powers are activated at the end of a combat phase, after damage from all attacks has been resolved.
In order to activate a power at the end of a phase, you must have taken no actions other than Evade. The Evade does not have to have been successful. The evade simply is your attempt to avoid damage before you activate your discipline.
Example:
John Tremere is being attacked by Jane Brujah. John is in serious danger of getting pounded flat, so he wants to use his Dominate. He announces his intention to Dominate and Evades.
The combat test is resolved, and John Tremere takes 5 levels of Damage. Ow. But he can now resolve his Dominate against Jane Brujah (taking into account wound penalties). Another combat phase will occur after the Dominate is resolved.
