Spouti™ stays in place via 2 factors: Support from the labia which form a cradle and supportive pressure from the thighs. The shape of the tab keeps Spouti™ from shifting to the right or left. These combine to allows Spouti™ to remain in place without shifting, during most physical activities.
We also have a couple of great videos on YouTube that offer some visual explanations!
This video discusses how to put Spouti™ on:
This video goes over some of the specs: