Thumbcuffs are a metal restraining device that lock thumbs in close proximity.
Thumbcuffs may also be used as toecuffs by locking the large toes in close proximity.
The standard universal handcuff key does not generally work with thumbcuffs, as most thumbcuffs use a smaller key.
Thumbcuffs with double locks have a lockspring which when engaged, usually using the top of the key, stops the thumbcuff from ratcheting tighter and prevents the subject from tightening them, possibly causing injury. Double locks also make picking the locks more difficult.
Note: as with all bondage activities, care must be taken. All the dangers associated with physical restraint apply to the use of thumbcuffs. No-one should ever be left alone when restrained. You are responsible for your own and your partner's health. See also safe, sane, and consensual.