Slipstreaming is when a rider uses the wake of the riders in front to gain a little speed. In Flamme Rouge a rider can move forward a square, for free, if they are close behind other riders.
Slipstreaming is applied from the rearmost rider (ie the one furthest from the finish line) then moving forward until the front of the race. It is possible for a rider to benefit from slipstreaming more than once in a single turn.
If a pack (a group of riders adjacent to each other, or a single rider) has exactly a single empty square between them and the pack in front of them, the whole pack can move forward one square, merging with the pack in front.
Consider the example below:
The rearmost pack, pack 1, has two empty squares between it and pack 2. They can not benefit from slipstreaming.
Pack 2 has exactly one empty square between it and pack 3, so they may slipstream. Both riders of pack 2 move forward one square, into the empty space, merging with pack 3.
Pack 2 and 3 have no merged into pack A. Pack A has one empty sqaure between it and pack 4, so pack A may slipstream. The whole pack moves forwards, merging it with pack 4 to make merged pack B.
Merged pack B is now at the lead of the race, there are no packs in front of it so it may not further use slipstreaming.