A Standby Stint means being available on short notice to fill a last minute cancellation. If you accept a Standby Stint you are essentially confirming that between those times, you are available and ready to leave at any point to do a Stint if needed.
If you turn down a Stint offer between the times of your Standby Stint, you will not be eligible for payment and it will be treated the same as a late cancellation for a regular Stint.