What is a handcuff in fantasy football?
A handcuff is the backup running back directly behind a starter, stashed on your bench so you keep the production if the starter gets hurt.
A handcuff is the next man up behind your starting running back — you draft or hold him so an injury to your starter doesn't sink your team.
If you own a feature back who handles 20+ touches a game, his backup would likely absorb most of that volume if he went down. Rostering that backup as a handcuff insures your investment. Some managers also stash opponents' handcuffs that have standalone, league-winning upside.
Handcuffing is most valuable at running back, where workloads are concentrated and one injury can swing a backup from irrelevant to a weekly RB1. Whether to spend a bench spot on one depends on how thin the depth chart is behind your starter.