After you get the 4 navs on top, the last one is outside red base. You can get that nav and go down at red, or you can keep going the same direction (counter-clockwise) and go down at blue. You can't turn at the red base nav and go clockwise back against traffic on top to blue base. The only reason you'd turn around and go "backwards", since it slows you down just a little bit to turn, is that it gives you an opportunity to make more contact with the enemy, and probably surprise them as they won't expect it. I guess we feel there is plenty of contact on the map already without allowing this tactic. I think it's been asked before, you might want to check through the QFTO - I think Booyah answered this a few months ago. [ Edit: lazy kiwi, here's the thread
Is this Legal? ]
Infinity - at map start you are not allowed to get in a hog on one side of the base and drive around the back of the base behind enemy hogs to surprise and kill them. It's not going to your nav. You can go behind if you are alone and avoiding getting shot or naded, or to pick up a teammate on the other side.
On Sidewinder, in scrims we are not allowed to drive to first nav by going under the bridge by 2nd nav. We also aren't allowed to drive along the cliff edge by 3rd nav, or to cross over the ice into the other side. We have to go straight to 1st nav. The reason being that if you go wide your gunner still has easy shots at 1st nav where everyone else is headed, but you are fairly safe on the edge of the map. This is not a public server rule, but it's really unfair to have 5 hogs converging on 1st nav taking each other out, and one hog sneaking under the bridge and taking pot shots at everyone else who's already at the nav. Any action that slows you down from first nav so you can take advantage of everyone else racing to that nav is unfair in my book. I know unfair to me might seem like brilliant strategy to someone else. I wouldn't warn New001 about it, but I think I've mentioned it to you before, not sure if you were driving or gunning, so hopefully you'll undersand my reasoning. As I say it's a scrim rule, so not enforced in public servers.
The same goes with people driving way out in the water on Death Island. You can shoot at first nav, knowing all other hogs are going to that point of the map and will likely run into your rockets. They're not looking for hogs out at the edge of the map (who aren't really going straight to their navs). While you're practically invisible out in the water, hard to hit, and you can easily dodge any incoming rockets.
The theme with all of the above is you have to be going straight to your nav without delaying to get an advantage or taking an unexpected route that is in fact longer. I know the one you described on Gephy is not a big difference which way you go, so that one is more of an arbitrary ruling - we just don't see the need for people to spin on the nav and start going backwards.
