The Shaft Junction does exactly what it sounds like. In 'merge' mode, it will take two engines' outputs and combine them, outputting the sum of the torques at the input speed. Note that trying to combine inputs of different speeds will merely result in a shower of sparks. In 'split' mode, they will take an input and send some torque each way at the input speed. To swap modes, shift-right click with the screwdriver.
The Shaft Junction requires 2 HSLA Steel Ingots, 3 Shaft Units, 1 HSLA Steel Gear and 3 Base Panels with the base plates in all 3 right slots, gear in the middle, shaft units above, next to and below the gear and the steel ingots in the top and bottom left corners in the Worktable to produce 2 Shaft Junctions.
(Picture needed for this section)
As stated in the description the Shaft Junction is capable of combining engine inputs and outputting the sum of their torques at the same speed or taking 1 engine and splitting it off into two directions at reduced torques but the same speed.
'Merge' mode simply takes 2 input of the same speed and outputs their combined torque. Trying to combine differing speed just results in the Junction failing to work.
When in 'split' mode simply right-click on the Shaft Junction to change how much torque is sent in the different directions. This works on a power of 2 system, 32,16,8,4,2. Terms to be aware of, Inline is across from the input and Bend is the side. A 15:1 Inline setting will send 15/16 of the torque out the side opposite the engine whereas the 1:7 Bend setting will send 3/4 of the torque out the side of the Shaft Junction. The 1:1 setting will send 1/2 of the torque out each side of the Shaft Junction.
(Picture of the GUI is needed to show what is being talked about.)