From Wikipedia:
"Ron & Kelly are Yielded by Rob & Amber (Season 7)The Yield, which was introduced in Season 5, allows any one team to force another team to stop racing for a predetermined amount of time. From Season 6 onwards, when there was going to be a Yield, the teams would find the message "Caution, Yield Ahead" in their clue. To Yield an opponent, a team places the picture of the team they wish to yield onto the Yield sign (found near one of the Route Markers). When the yielded team arrives at the Yield, they must turn over an hourglass found on the Yield sign and wait for all the sand to drain before continuing. Each team gets an envelope with their 'courtesy of' sticker at the start of the race (which goes on the bottom-right corner of the Yield sign), and if a team's envelope goes missing, that team loses all power of using any future Yields. [8] Teams like Linda & Karen (Season 5) and Joseph & Monica (Season 9) could not use the Yield, since they had each lost their envelope throughout the course of their respective Races.
Each team is required to stop at the Yield station, and state if they are Yielded or not, and then if they are using the Yield or not to the camera. In the early use of the Yield, teams were required to remove a numbered marker as well when they performed this aspect, though beyond tracking their placement in the leg, these appeared to have no additional purpose.
In Season 5, when the Yield was first introduced, the sign was larger than the future series of the Yield. Teams weren't aware from the clues whether or not there was a yield ahead. In addition, the teams didn't have to pull a number for priority, only when two or more teams were fighting to choose whether to yield a team or not. Only this happened once when Chip and Kim grabbed a number as they were aware Kami and Karli were approaching and wanted reserve in case they wanted to yield Kami and Karli or not. However, when they realized they were the last two teams, rules changed so that all teams were aware of their positions.
Like the Fast Forward, each team may use only one Yield during the game, and only one team may use each Yield. However, each team may be Yielded by other teams an unlimited number of times. From Season 6 to Season 7, the number of Yields totaled three in the entire race, instead of one on every leg. Since Season 6, teams have been warned about an upcoming Yield in the clue immediately preceding it. Like with Fast Forwards, Yields that are present in the race but aren't used and have no affect on the outcome are not aired or mentioned. For example, Season 8 mentioned only having two Yields available on the entire race, but a third Yield on Leg 1 (New York-Pennsylvania) went unaired and unmentioned."
