I'm wondering what the overboost solenoid does when the boost passes its pre-set limit?

I ask because twice now while doing some serious highway cruising, my wastegate hose had popped off and didn't realize until I got home (the first time at least, the second time I noticed the turbo whine.) I'm sure I was getting full boost for minutes at a time going up mountain passes and such. So what would the solenoid do? Just regulate at 15 PSI or what?

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1983 300SD