The Post Sports Live crew debates which single player is most important to the Nationals’ overall success in 2014. (Post Sports Live/The Washington Post)

In baseball, not all plays are created equal. Thanks to recent computer analysis, we know precisely how much any possible play changes a team’s chance of victory. More than half of all plays change those odds by two percent or less. Plenty of plays rounds off to “0 percent.”

But other plays are cataclysmic by comparison. Daniel Descalso’s single off the Washington Nationals’ Drew Storen in Game 5 of the 2012 National League Division Series increased the St. Louis Cardinals’ chances of winning from 13 percent to 50 percent.

In the legendary Game 6 of the 2011 World Series, four key plays from the ninth through the 11th innings changed the probability of a victory by 54, 47, 43 and 37 percent. “Winning” one such play matters enormously while the outcome of a dozen other plays combined may hardly matter at all.

That’s why baseball’s new vastly expanded replay system is such a major improvement. How can a modern sport allow a “47 percent play” to be wrong? It just can’t. But there’s also a huge delicious bonus. Baseball, the sport that adores the second-guess, now has a rich fresh arena for teams and fans to debate an entirely new area of strategy: What’s worth challenging, what’s not and how the heck do you decide?

Baseball has not had such a basic change in game tactics since — ever.

Twelve-year-old Drake LaRoche might not have an actual contract with the Washington Nationals, but the team has considered him one of their own ever since he started showing up at spring training with his father Adam LaRoche six years ago. (Whitney Leaming/The Washington Post)

So far, nobody has more than an inkling of what’s smart, what’s dumb and who’s going to figure out the Challenge Edge first. Yummy.

MLB had a brilliant idea with its new system. It’s so perfect you wonder if it was an accident. A team is guaranteed only one challenge for a whole game — just one precious challenge. If you get that challenge correct, then you get a second challenge. But that’s all — there’s never a third.

Do you challenge a call in the third inning, when your chance of winning changes by just a half-percent, and risk a mistake that may haunt you the rest of the game? What if, in extra innings, you have no ability to demand a replay of a three-run play — perhaps a diving catch of a bases-loaded flyball with two outs in a 4-2 game that’s ruled a “trap” not a catch?

Under the new rules, all you can do is beg the crew chief to ask for a replay himself. He can. I suspect he probably would. But what if he didn’t? What if he thought he saw the play clearly? What if he hates your guts as much as Ron Luciano hated Earl Weaver? (It happens.)

In many close games this season, one team will be out of challenges while the other still has its challenge and, potentially, another one, too. That is going to feel like an advantage, whether it ever becomes a factor or not. Will it play on the minds of the teams? Is there added psychological value in holding back your challenge even if you have a base stealer incorrectly called out in the fourth inning? Or will the failure to challenge be seen as gutless?

While expanded replay will get more calls correct and make baseball seem up to date, its biggest value may be that the strategy around using the one guaranteed challenge will be seen as a major new element in a game that thought it was fully formed a century ago.

For reference, how many MLB plays are just not very determinative? In the Nats-Cards Game 5, 33 of 88 plate appearances changed the winner’s odds by one percent or less. Another 14 plays nudged it by only two percent.

My guess is at least half the plays in a typical game aren’t worth challenging unless you are virtually certain of winning. Remember: The video replay team that will review all plays from a central studio in New York (nicknamed “BAM,” for Baseball Advanced Media) may lean toward “inconclusive.” Will they really reverse all the possibly “wrong” calls decided by an inch or two? Isn’t this system’s purpose to prevent miscarriages of justice — not to nitpick?

“Since lights were put in our stadiums, this is probably the most historic development in our game,” said Atlanta Braves President John Schuerholz, one of the key members of the committee that crafted the current system.

Maybe that’s over the top. In practice, maybe umps will initiate replays, out of a sense of fairness, for teams that are out of challenges. Maybe this, maybe that — and that’s the tease. For a few more weeks, we don’t know.

This week the new expanded replay rules have been used with major leaguers for the first time in selected exhibition games. Everybody’s first reaction was to time it. If expanded replay is a pace-of-play killer, it probably shouldn’t survive. So far, with limited data, managers use less than 40 seconds to ask for a review; the whole process has required about three minutes. That needs to improve.

Schuerholz thinks total challenge time will fall to 60 to 90 seconds in the regular season. Why? The spring training system in dinky exhibition parks, with walkie-talkies to replay trucks, is so last century. The BAM unit will be studying close plays even before the manager steps on the field and long before he actually challenges. By the time umps can ask replay central, “What have you got?” the answer often should be available.

Baseball expanded its replay system to improve its product, to defuse criticism that it’s stuck in the past and to avoid infamous mistakes like wrong-by-a-mile calls that probably cost St. Louis a World Series in ’85 and cost Armando Galarraga a perfect game in ’10.

But the greatest benefit to the game — and to our fun — may be the addition of the second-guess of the challenge. You can almost hear the hubbub, as video screens in parks will be allowed to show controversial replays.

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