look_ang is the offset that is added to the (yaw or left-right) viewing angle. In the original game it is in effect when using the "look around shoulder" commands.
Negative values offset the camera angle to the left, positive ones to the right.
Because the offset is reset towards 0 at a rate proportional to itself (currently hardcoded to one fourth), there is a point where the constant addition per time unit (+- 152, unless overridden by EVENT_LOOKLEFT/EVENT_LOOKRIGHT) and the resetting balance each other out. With the hardcoded values, these are -605 and 608, that is, about more than 90 degrees.