What is the test for an agreement between a plaintiff and a defendant not to sue?

In Pellett v. Sonotone Corporation, 26 Cal.2d 705, 160 P.2d 783, 160 A.L.R. 863, the agreement involved was that a certain joint tort-feasor defendant would defend the action until judgment and that the plaintiff, in the event of success in the action, would not levy execution upon any of the property of that certain defenant. It was neither a release nor a covenant not to sue and there was no dismissal of the action as to the defendant. In that decision, however, Mr. Chief Justice Gibson, speaking for the court, 26 Cal.2d at page 711, 160 P.2d at page 786, said: