I should have expected the help file to be pathetically vague.
Slopes are always computed when an object is part of a model even if the slope parameter is set to none.
I'll try to explain without going into the details. Normally you don't see the slopes of an object unless you are viewing it in context with a model since slopes are only computed when merged into a model. However there are times where you want to see how the the slopes are being computed without showing the entire model. That is what the toggle does. It shows the slopes of the object even when the model is not displayed.