These leashes help prevent you from losing your fins during the session. To put them on, first put your foot in the leash then in the fin, before tightening the ankle fastener.
The fin may come off during the session, but it will remain attached to your ankle by the leash. You will then be able to put it back on.
If you want a leash that stops the fin from coming off, choose the 900 fin leashes.