Ben Tate of the Houston Texans, Kendall Hunter of the San Francisco 49ers and Jason Avant of the Philadelphia Eagles lead the list of key handcuffs this week.
An injury to a key player on your team at this point of the season can be deadly to your playoff prospects. So you need to handcuff to protect your team.
Typically we look for a top producer whose backup on his real team is a borderline producer and is therefore probably available in your league. For example, if you own LeSean McCoy, you would like to handcuff him. But it you look at the Eagles' depth chart, you can't find anyone that would be a reasonable replacement. So in that case, you'd be better off just hunting the waiver wire if McCoy suffers an injury.
Now in the case of Calvin Johnson, you can find a rookie on his team who had 150 catches and 23 touchdowns in his last two years of college. Aha! There is your handcuff.