Tracker

Tracker
Tracker is a block that allows orientating the body with mouse.

The setup defines the margin between the exact aiming direction and the tracker's direction. For example, 0 means the tracker will turn the body directly towards the aiming direction, 10 means it will be additionally raised by 10 degrees. That is helpful for the weaponry: players usually aim a bit lower to see things in the front, so the weapons would fire at the ground with a default setup.

Trackers cannot rotate large bodies as if you've tried to make a plane rotating towards the aiming direction. They can only rotate bodies relatively to their joints: place a rotor on a body, then place a tracker on it, set it up and the body on the rotor will rotate. The non-square half defines the tracker's fore side, keep this in mind when placing it.

Trackers can rotate bodies placed on rotors and hinges.

Rotating the whole machine
Trackers are an interesting thing to experiment with. You can actually rotate your machines just with trackers, though it requires artificial structures, such as this: Here, trackers are placed as additional bodies, connected via rotors (rotors have "Spring - 100, Damper - 100" setup). Paint this transparent and connect to your machine's center of mass through the chassis block in the back.

Basically, the trackers try to rotate their bodies towards the mouse but are stuck with the others. That causes the structure to create a controlled continuous shake that affects the whole machine and rotates it.