At third base in the eighth inning, the speedy pinch-runner peered over to Reds reliever Nick Masset, who focused his attention to the mound at his feet.
Nelson stepped toward home as Masset looked in at Rockies catcher Miguel Olivo at the plate.
Done inching, Nelson took off.
"(Massset) was looking at the ground," said Nelson, who saw an opportunity to score, and took it upon himself to dash home. "He wasn't paying attention. (Reds third baseman Scott) Rolen was playing me back. I just decided to go and didn't stop."
Nelson added: "That wasn't the plan, but it worked out."
Hearing his teammates call to the commotion on the basepaths, Masset turned his attention -- and his body -- away from the plate in front of him.
The right-hander was left without a play, and the Reds have left Denver without a win for the second year in a row, following the Rockies' remarkable 6-5 comeback victory on Thursday at Coors Field.
"Never stole home before, never tried," Nelson said of what was also his first big league steal. "Won't forget that one."
In a matter of 90 feet and the blink of an eye, Nelson turned a mistake into a milestone. The suicide squeeze had been called, and Olivo was prepared to plate the go-ahead run himself.
"I was gonna squeeze the ball anywhere the ball comes," the Rockies catcher said. "Then I saw Nelson coming and was like, 'What are you doing?' Thank God [Masset] turned off the wrong way."
From the dugout, Rockies manager Jim Tracy was left wondering how such risk evolved into such reward. It was the 15th time in franchise history a Rockies player stole home, resulting in Colorado's 23rd come-from-behind win of the year.
"At first, it was a little bit more than, 'Oh, no,'" Tracy said with a laugh. "Then, as he was coming down the line, the realization was, 'He's not going to get him. He has no chance to get him.'"
The consequences are not lost in the moment. The eighth-inning rally -- sparked by shortstop Troy Tulowitzki's 18th homer of the season -- claimed the lead after Jason Hammel fell behind, 5-0, in the second. To Hammel's credit, he faced just 18 batters in the following five frames to keep it close.
"To go out there and maintain his focus and put five zeroes up there and give us a chance offensively to chip away and get ourselves back in the game -- that's a huge, huge difference in the game," Tracy said.
Colorado has won a season-high seven in a row and 19 of its last 22 against the Reds at Coors Field. In the last three weeks, the Rockies have swept the Braves, Padres and Reds -- all leading their respective National League divisions at the time.
So, here we go again. The namesake of Rocktober's already been used, so what do we call this? Rocktember?
"I've been a part of some crazy games," Tulowitzki said, "But that one was right there at the top."
Masset, a sturdy mainstay in the Reds' bullpen this season, had not allowed a run since Aug. 4. It's unlikely he'll ever see one like he did in Thursday's eighth inning.
"Before I was about to throw that pitch, I was going to fake to third and go to first," Masset said. "Instead, he took off right before I had the chance. That's why I thought the runner from first was stealing second.
"It's one of those things you think will never happen and it happened."
Nelson, a stranger to the Rockies this season -- Thursday was just his seventh appearance of the season, having spent the majority of his season with Triple-A Colorado Springs -- is no stranger to home-plate dramatics.
In his first Major League at-bat on June 19, he reached base, aided in the go-ahead scoring and crossed the plate for his first big league run.
Thursday ought to be another memory that won't soon escape him.
The Rox have dominated the Reds at Coors Field for years, not having lost to Cincinnati in Denver since Aug. 23, 2008. The Rockies outscored the Reds, 29-15, this week.
But on Thursday, without a doubt, they stole one.
Joey Nowak is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.