Every archer you see shooting in the Olympic Games in Rio de Janeiro this summer will have a device on their bow called a plunger, or cushion plunger. It’s a small piece of gear that kind of looks like a spark plug, which works in concert with the arrow rest.

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And it’s absolutely critical to accuracy.

The plunger is mounted through a hole in the riser, just above the shelf. When an arrow is nocked on the string, the shaft will sit on the rest arm and press against the plunger tip.

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What does it do?

Well, arrows released with fingers flex laterally as they leave the bow. The flexing is what enables them to clear the bow and stay centered as they come off the bow string.

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Think of the plunger as a shock absorber for your car. The shocks soften bumps as you drive over them, and keep the car from bouncing off its line.

A plunger cushions the flexing of an arrow as it leaves the bow, to promote consistent arrow flight. (In the video below, notice the initial flex of the arrow is in toward the riser. You will see the arrow stays centered as the plunger cushions that first, inward bend.)

The tip of the plunger is spring loaded, and so it gives as the flexing shaft presses against it.

The tension of that spring is adjustable, and it’s critical the archer gets it right for his or her setup.  Arrows that are spined a little on the stiff side require a weaker plunger to absorb the shaft flexing, but if the plunger is too weak, the plunger will be collapsed by the clicker, and the arrow could hit the riser upon the shot. Arrows that are spined a little on the weak side require a stiffer plunger setting to counteract any excess flexing by the arrow.

Olympic archers employ many methods to fine tune their plungers to the get the most accurate and forgiving setups. How an individual archer sets the plunger in Rio could mean the difference between watching the medal rounds from the stands or winning the gold.