Now things are getting pretty wobbly in there. This tightens all that up. The overdrive support.
This thing not only acts as a center support but it also collects and diverts pressure via those 4 holes at the bottom to several different pistons as well as housing the overdrive piston itself. You just drop it back in after changing those 2 scarf cut sealing rings. I torque the bolts after the trans is completely assembled, just don't forget.