If you are placed on Probation in New York, the Judge will issue you an Order which sets forth the requirements you must meet and the Probation Department will enforce them. If you fail to abide by the Judge's Order, then the Probation Officer, PO, will violate your terms of Probation and you will be required to appear back before the same Judge who originally issued the Order. When you appear before the Judge, you can either admit or deny the charges. If you admit them, the Judge then can and will resentence you. If you are remorseful and its only a minor violation, then the Judge may put you back on Probation for the rest of your term. Or he/she may give you some time in jail and then return you to Probation. Or the Judge could simply change your sentence to jail time and then discharge you from Probation.
If you deny the charges, you then have the right to a Hearing, not a trial, to determine if you truly violated the terms of your Probation. Unfortunately, the level of proof in such a hearing is simply a preponderance of the evidence, rather than beyond a reasonable doubt as it would be during a trial, so the proof doesn't have to rise to a very high level. In most instances, the defendant loses VOP hearings and then is sentenced by the very judge who heard the testimony. That's why may VOP cases resolve themselves by a plea to the violation and throwing themselves on the mercy of the court.