[TwilightsCall]

Originally Posted by bhl88

Spoiler:

Cut off the Master's arm (the one with the command spell).

Spoiler:

In addition, a servant that has no current master (like Saber in UBW) can make a contract at will with anyone who has a Command Spell.

As far as Caster, I think her relationship with Kuzuki was a bit different then the normal Master/Servant relationship, since (theoretically) Kuzuki didn't have a Command Spell, so she had to replenish her magical energy by...other means

There appears to be something else at work as well, since Caster was able to force Saber in to a contract, as well as Kotomine with Lancer, though I'm not familiar with the situation under which he took Lancer's contract (whether Lancer entered it willingly after his master was killed or if it was forced on him).