Seems to me the arrow simply means the car is going quicker than the rest. Not just at a single point, but on the whole, perhaps over the past few seconds or so. Thus, it's likely to appear when drafting/overtaking, but can also appear when the car is out on its own and, say, clears a chicane particularly well.
JohnBM01
-THE CONCEPT OF DRAFTING-
When race cars zoom down a track, the cars generate a good deal of turbulence. The higher the speed, the more the turbulence. When a car behind another tends to get sucked into the slipstream of another car, it has a sort of vacuum effect. When the racer behind gets just enough pull from the other car's slipstream, the trailing racer can easily zoom past the car ahead with the slingshot. Drafting is simply when you have one racer ahead at speed and has a sort of turbulence that suddenly pulls you win. And when you have enough turbulence, you can inch closer and closer until you simply blow past the car ahead. The slipstream is that turbulence behind a leading car.
Or set me straight if I'm wrong.
Well, you're half right.
Yes, drafting definitely gives you a speed advantage, but it's not because of turbulence 'sucking' you along. Rather, the air behind the car in front is both a) lower pressure and density, and b) travelling at roughly your speed and in the same direction that you are travelling in. The first factor means slightly less air to deal with, which contributes in part to the second, far greater factor means that there is much less wind resistance on your car.
With less force pushing back on you, your car is able to accelerate faster and reach higher speeds than it can out in open air. It's not a force pulling you along, it's a diminished force pushing back that's doing the business.
Nonetheless, and physics aside, it all comes together as drafting, and it's an essential racing tactic. On that score, you're spot on the money and I'm sure it's helped your racing.
