"Joint suspension" is jailhouse slang for probation with a suspended prison sentence.
There are two ways to put somebody on felony probation. The first, and is most common, is by suspending imposition of sentence, which means the judge will make a decision about prison time if you violate.
The second is by imposing a prison term, but suspending imposition. That means the judge say, "I sentence you to state prison for XX years," but you don't actually get on the Department of Corrections bus unless you violate probation.
Judges usually impose a suspended sentence to send a message to the defendant that he's on thin ice and will probably go to prison for the next violation. Some felonies, known as "wobblers," can be reduced to misdemeanors after probation, but a felony can't be reduced if the judge imposes a prison sentence, even if the sentence is suspended.
Since it involves probation, it only happens after a person is convicted, either by a plea or through a trial.
