In fantasy football, a handcuff is a backup running back (or sometimes another position) who plays behind one of your own starters. The idea is simple: if your starter goes down with an injury, the handcuff steps into a bigger role and your team keeps scoring. The best handcuffs are backups on high-scoring offenses who would take over almost all of the work if the starter missed time.
Handcuffs matter most at running back because of injuries and workload. Most fantasy rosters have bench spots, and using one on a quality backup is like insurance for your best players. Common targets include the backup to a top-ten running back, or a backup on a team that loves to run the ball.
There is a tradeoff, though. A handcuff usually scores few points while the starter is healthy, so that bench spot could go to someone with a better chance to help you every week. Many managers only handcuff their most important running backs, or wait until late in drafts to grab them, so the cost stays low while the protection stays valuable.