Apart from clutches and clutch-like elements, Simscape™ Driveline™ simulation is a special case of Simscape simulation.
For information on fixing simulation errors, see Troubleshoot Driveline Modeling and Simulation Issues.
On how Simscape models work, see:
During simulation, Simscape Driveline software checks the clutch and clutch-like blocks in your model for locking and unlocking events. If a locked clutch meets the criteria for unlocking, or an unlocked clutch the criteria for locking, the respective clutch states change.
If one or more clutch and clutch-like constraints change, the driveline states are repartitioned into new sets of dependent and independent states. The repartitioning requires a partial reinitialization of the driveline that preserves the state of the driveline before the clutch changes, except for the subset of constraints and states affected by the clutch transitions.