http://www.lawdepot.com/contracts/sublet-agreement/faq/
The whole idea is, instead of only having two people responsible for rent, after signing the lease, there will be a third person who is also legally responsible for the rent.
Assuming A moved away, B stays, and A found C to sublease. A proper sublease would make A, B, C all responsible for full rent, the landlord would not care how they divide up the money themselves. Even though A moved away, he signed the lease original and he should not be allowed to get rid of the responsibility.
