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 adjustment tanka (plastic part) should be positioned between the ankle and the fin.
The fin may come off during the session, but it will remain attached to your ankle by the leash.
To prevent the fin from coming off, opt for the leash for the 900 fins.