How do track relay handoffs work?

Few events in the Olympics give athletes more chances to screw up than the 4x100 relay. As we saw during the women's qualifying Thursday when the U.S. fumbled its baton after a collision between Allyson Felix and a Brazilian runner, the relay can be fraught with risk during those brief moments when the baton is being passed from one runner to another.

The Americans have been given a reprieve after qualifying with the final during a time trial Thursday night, but the mishaps earlier in the day showed why this race can be so difficult. Even with the U.S. into the final, two different teams (Kazakhstan and Brazil) had their times disqualified in the heat for violations. Over the years, Team USA has had its own troubles in this event, too.

There are specific rules for how the relay works and passing the baton isn't as easy as it might look, especially when the pressure is on in the Olympics. Before the men's and women's 4x100 finals Friday night, here's what you need to know about the relay.

The "changeover box"

The basic structure of the 4x100 relay involves four sprinters per team running one at a time in a single lane, with each athlete doing a 100-meter leg. There's a special area on the track where you're required to hand off the baton to the next runner known as the "changeover box."

The changeover box is a 20-meter area that's situated right at the starting line of the leg. It extends 10 meters before and 10 meters after the starting line. This is where Felix got bumped during qualifying Thursday as she was about to hand off the baton to teammate English Gardner.

Before the "changeover box," the runner for the next leg has 10 meters in order to build speed. The idea is that the runners of the current leg and the upcoming leg will meet near full speed inside the changeover box, at which point they'll attempt to pass the baton.

The blind handoff

Part of what makes passing off the baton so difficult is that it's usually done blindly, with the leading runner reaching backwards to be handed the baton before completing the leg. This is all done at speeds of up to 20 mph and you cannot leave your lane, so it's understandably difficult.

Much of the responsibility for the blind handoff goes to the incoming runner. That athlete will typically make a verbal sign when closing in on the changeover box, which signals to the next runner, who never looks backwards, to be prepared for the handoff. It's on the incoming runner to make sure the exchange goes properly by placing the baton into the next runner's outstretched hand.

Doing these handoffs effectively is incredibly important to the relay, given how closely the world's best teams perform. The difference between gold and fourth place can be small, and a perfect handoff can help shave time off just as much as raw speed. The best teams combine that pure physical talent with masterful handoffs that allow each runner to start and finish with momentum. And, yes, you can throw it.

What if you drop the baton or commit a violation?

When things go wrong, like we saw for Team USA in their first qualifying run, it can doom a team. Even if you're not disqualified, a dropped baton usually means you're finishing in dead last.

A team is allowed to continue racing after it drops the baton, which can be important when there's a potential appeal. When the U.S. women fumbled the baton in qualifying, Felix was the one to quickly realize they needed to finish the race if they wanted to file a protest. She got Gardner to go pick up the baton and complete the race, as they would've been automatically disqualified if they didn't.

Beyond not finishing the race, there are other ways to get disqualified. For example, a team is out of the race if it hands off the baton outside of the changeover box, veers outside of its lane or obstructs with another runner. In women's qualifying, Brazil was DQ'd for obstruction and Kazakhstan for lane infringement. And if someone merely drops the baton, they're not disqualified, but good luck rallying to win a medal.

A recent history of baton problems

When Felix dropped the baton in qualifying on Thursday, it likely gave some flashbacks to members of the U.S. Olympic Committee. This had been an issue for the Americans in 2004 and 2008, which led USA Track & Field to give a "comprehensive review" of its relay program before the London Games.

In 2004, the U.S. men won the silver medal after the British team passed them at the last second -- winning by 0.01 seconds -- due to some poor handoffs. The women, meanwhile, were disqualified after Marion Jones failed to hand the baton off to teammate Lauryn Williams inside the changeover box. Four years later, the U.S. men and women didn't even make their respective finals after botched handoffs disqualified each team in the heats.

The U.S. got back to its winning ways in 2012, earning gold for the women and silver for the men (although the latter has since been revoked as part of Tyson Gay's doping punishment). But for a program that had re-committed to doing the relay the right way years ago, then proved it had done so in London, seeing the dropped baton in Rio definitely had to give some Americans chills.

All of these possibilities make the baton handoff, and the relay itself, both challenging and exciting to watch. The women will race at 9:15 p.m. ET and the men at 9:35 p.m. ET. As long as they don't drop the baton, both teams should have good shots at medals. You can stream both online via NBCOlympics.com or watch them on NBC.

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U.S. women's 4x100m relay team's odd qualification