Swivels allow a bird to change position on a perch or move on a leash without getting tied up in knots.
The ball-bearing swivels (Often known by the brand leading name of ‘Sampo’) by design work better under pressure and are generally considered Stronger and lighter than there ‘D’ Type equivalent size. One of their few draw backs is that there is no way to visually inspect the condition of the working mechanism. For this reason it is recommended to swap them out more frequently than the D Type
Below is a diagram for Swivel fitting. The leash should attach to the small end of a Ball Bearing Swivel and the ‘U’ section of a D type swivel. Conversely The Larger Ring of the BBS and the ‘O’ of the D type should be attached to the jesses
A light oiling and frequent inspections should avoid most problems. If in doubt buy 1 size bigger than the minimum size for your bird (Just to be sure!)