[AlftanSr]

Squaring is a funny thing. All of your inputs are between -1 and 1.

If: -1 < x < 1
then: x > x^2

If you do away with squaring, the following calculations will be easier to debug:

Speed[NW] = y + x + twist;
Speed[NE] = y - x - twist;
Speed[SW] = y - x + twist;
Speed[SE] = y + x - twist;