Normally, the mechanical compression releases are on the cam. They are generally some sort of spring loaded bumps close to the bottom of the base circle of the exhaust lobe of the cam. This slightly opens the exhaust valve for an instant as the piston moves up on the compression cycle. The amount of air/fuel to be compressed is reduced so the starter can more easily roll over the top of the compression stroke. This builds momentum which stored in the flywheel over the remaining three cycles. The cam's increased RPM during this rotation causes the spring to collapse, disengaging the compression release for the next compression cycle. At that point, ignition should kick in and the engine starts. That's the gist of it anyway. Some points may vary from one engine to the next.
If you remove the mechanical compression release, it is necessary to manually roll your engine over the compression cycle and then hit the starter. This gives the starter a running start at the next compression cycle and hopefully you build enough momentum to where it will make it to TDC and start the engine.
On a V-twin, this is probably a little different, but the principle is the same.
Since Both pistons ride on the same crank journal, you have to roll it past the second cylinder's compression cycle.
