The 'Dakar Spec' Countdown Method
There are probably as many ways to using one's fingers to do a countdown as there are languages. Provided below is the method used at the world famous Dakar Rally which is two full weeks of racing across South America covering thousands of miles. Coordinating details such as these across hundreds of volunteers must be just one more item in a long list of training sessions for those lucky enough to witness and help with the Dakar Rally.
This particular method is interesting in that the hand positions are optimized such that each position has complete clarity to the racer. It can't be claimed that it is the simplest method for the volunteer, but with a little practice it is very easy.
Interestingly, these correspond with the FSL (French Sign Language) set of numbers, as the Dakar is run by the ASO, a French organization. The ASL (American Sign Language) numbers are the same.