Switching from FA off to on leaves the throttle who knows where on a Xbox controller
Where it sets the throttle when you re-engage will always depend on your momentum.
Based on the orientation of your ship, if you are traveling backwards it will set the throttle to reverse - in line with how fast you were traveling that direction.
One thing to note is that the speed boosts in the maps can be quite neat to experiment with... they will always give a huge speed boost according to your throttle setting when flying through it.
For example... you can boost toward a speed boost powerup and set your throttle to full reverse before hitting it... then super boost backwards. This is fun if someone is chasing you and can allow you to reverse the situation.
The speed powerups will also effect FAOff maneuvers... so for instance... fly/boost through and toggle off FA and then use vertical thrusters for a super vertical zoom, etc.
Ice Field is a good map to test on because it is so large and it has two of the speed powerups.
At any rate... glad u are having a good time... all the best on your flights! o7