The Shaft Junction can be used in two ways: It can connect to two Shaft Power transmitting blocks operating at equal Speeds, add their Torques together, and output the result, or it can connect to one Shaft Power transmitting block, split its Torque, and output a portion of the Torque in two different directions at equal speed. The Shaft Junction does not change the Speed of the power input or output, only the Torque. It can be used for two purposes: combining multiple weak engines to power a single powerful machine, or taking the output from one powerful engine and feeding it to many weak machine. The Shaft Junction's input/output side orientations can be changed with a Screwdriver.
When set to combine incoming Power, there are two limitations. First, the input speed of the two shafts must be equal. If two power sources of uneven speeds are connected to a Shaft Junction, it will emit sparks and soon explode. Second, a series of Shaft Junctions can only combine the inputs of four separate Engines. If more than four engines are connected, some of them will be ignored.
When set to split incoming Power, there is an interesting aspect to the Shaft Junction: the power they split does not need to be evenly distributed to both outputs. Shaft Junctions can split the Torque 1:1 (even), 1:3, 1:7, 1:15, or 1:31 between their two outputs. The output ratio can be set by shift+right-clicking with a Screwdriver.
Unlike most blocks that transmit Shaft Power, the Shaft Junction has no maximum Speed or Torque, and cannot be overloaded.