Mountain biking: lower-back pain in "attack position"
I'm don't think much of staments that any one position is “exactly the correct position”. Any position is wrong if you need to strain and stiffen yourself to keep it.
It's true that the “attack position” works well in a variety of riding situations, but there's no point keeping it all the time when there's not even something to attack in the trail! Especially for downhill, the more flexible&amp;free you can move around the bike, the better. Do default to “attack” in doubt on technical bits, but then relax again.
Yes, occasionally switching what foot is forward also helps and is a good thing to practice, but many (most?) mountainbikers always coast with the strong foot forward, so this should really not be necessary.
Also, bike setup can make a lot of difference. Try raising the bars a bit, softer rear suspension (if you have) or plusher rear tyre. And clipping into the pedals has many small advantages, especially when it comes to riding more relaxed, because you won't need to spend effort just to keep the feet on the pedals.