To clear something up once and for all according to Wikipedia:
The Fast Forward allows the team that receives it to skip all remaining tasks on that leg of the race and proceed directly to the Pit Stop. To receive the Fast Forward, the team must be the first team to perform and complete the task described on the Fast Forward clue. The clue is found along with a regular clue at one of the Route Markers.
Only one team may use each Fast Forward. Any team that is beaten to the Fast Forward will have wasted their time and must go back and pick up where they left off. Also, prior to Season 10, each team was allowed to use only one Fast Forward during the whole race, requiring teams to decide when it was most advantageous to use it. With the introduction of the Intersection route marker in Season 10, a team can claim two Fast Forwards in a single race, one while part of a grouped team during an Intersection, and another as a single team.
Originally, Fast Forwards were offered in every leg of the race (provided that there was as least one team left that had not used one), except for the final two legs. Starting in Season 5, the number of Fast Forwards was reduced to two on the entire race. In the event a Fast Forward was not attempted by any team on a leg, the task involved was not shown, though clue envelopes with the green Fast Forward marker can usually be spotted by the viewers.