The first time a ship is assigned a tractor beam token, the opposing player can force the ship to perform a boost or barrel roll using the 1-straight template. Is there a reason why you can't force a ship backwards?
It's not like there's a rule that the involuntary movement must bring the ship closer to the user of the ... in fact, if you hit a ship facing away with you, you can force it to boost away from you.
My theory is that the only reason it's not allowed is because this game doesn't have any backward movement (yet... Quadjumper may change that). It's easier to say "boost" on the rule card for Tractoring than it is to describe rules for moving in all four directions. Does anybody know for sure?