Guess I did manage another post before too long had passed since the last one. Weird...
So, well, considering I've managed to get somewhat of a reader base (as a consequence of linking here from my gaming club's forum) I guess it's worth stating that this will be old news to them. I'm going to write it anyway, but hey...
For most people, the intuitive way of moving a vehicle is (presumably; I haven't done the necessary research) to turn it and then move it straight ahead. This would make sense for a real-world vehicle, after all.
However, nowhere in the rules for vehicles (they're on page 57 of the Big Black Book, just in case you don't believe me) or indeed the general movement rules (p. 11) does it say anything about what direction to move a vehicle. This means that a vehicle can just as legally move sideways as straight ahead. Weird and unrealistic? Hell yeah. Is it what the rules say? Yup.
(I imagine all 41st millenium vehicles to be equipped with great pneumatic pistons that shoot down on one side, violently flipping the vehicle a full three-sixty through the air, to land right beside where it started, but I'm silly that way).
This might seem like a trifle, but on crowded tables, and with slight miscalculations in the planning stages, this is often the difference between a clear run and a difficult terrain test. In the worst of circumstances, it might even allow a hemmed-in vehicle to move where otherwise it couldn't.
One of the situations in which I've actually seen this be the difference between moving and not involved close combat. A massive mob of Orks charge a vehicle (I think it might have been a Land Raider, but any vehicle will do) and another unit nearby, locking the Orks in combat while surrounding one side, the front, and the back of the vehicle. In the vehicle's turn, no movement forward is possible (since you cannot Tank Shock enemies that are locked in combat) and turning faces the same problem. Playing by the forward-is-the-only-way-to-move rule, the vehicle is stuck.
If, however, we play by the rules as I read them (as always subjective at best), the vehicle can activate its jumper pistons and move out sideways. Is it a reasonable solution? I think so. True, it's silly to imagine a Land Raider jumping, but I find it equally silly to imagine the 'Raider driver remaining still just because there are orks in the way.
So, with the image of jumping Land Raiders still fresh in everybody's mind, that's it for me.
Tally ho, and all that.