Bevel Gears are used to transmit Shaft Power between two adjacent blocks. Unlike Shafts however, Bevel Gears are used to turn corners.
Bevel Gears are not rotated with a Screwdriver. Instead, they have a GUI through which their input and output directions are determined. The directions are not relative to how the block was placed; they are the absolute compass directions (North, South, East, West, Up, Down). Bevel Gears cannot transmit power in a straight line. Due to a minor rendering issue (and due to how gears work), the output shaft of a Bevel Gears block will visually turn the wrong way relative to the block it connects to.
Unlike most blocks that transmit Shaft Power, Bevel Gears have no maximum Speed or Torque, and cannot overload and break.