These give the bots a sense of touch, allowing them to tell when something has hit them or if they have hit something.
A good use of these are with Blind bots to give them some sense of what is going on around them.
.hit - Has your bot been hit by another bot?
.hitup - hit from the front?
.hitdn - hit from the back?
.hitdx - hit on the right?
.hitsx - hit on the left?
Please do not confuse the sysvars above with the sysvars below. ("Hit" does not mean hit with shot, it means hit by a physical robot. For shot look below.)
Note from the C++ front lines
In the 2.4 C++ port (probably will be called 2.5 I think) hitup, hitdn, hitsx, and hitdx will probably be replaced by hitang, which will return the angle of the hit relative to the eye.
The reasons are two fold: hitang would be more useful in practice, and free up 3 other sysvars (making the sysvars file less cluttered).