580CK what is purpose of the shift control lever ?

I have a 66-71 580 CC Loader backhoe with the power shuttle which to the left by the shuttle top cover is this U shaped Rod and as I look at the parts diagram I dont understand what is it for or do , and when does it move and why? Would that have anything to do with it jumping out of Hi/Low position . Also is it possibe for the range shifting fork for the shuttle to move on the shift rail ? It looks from diagram that the rail is driiled and theaded so I would think not ... or is it more chance of fork being bent ? or the gear worn out as when its in the forward postion it jumps out of gear . Also I added too much fluid to the power shuttle as I had a hard time reading the dip stick and put way too much in and then later I see it running out of the bellhousing where the clutch rod goes in ..eventually it quit dripping..the fluid I added was a trans-hydraulic fluid for use in all Case New Holland requiring CHN MAT 3526,3525,-JI Case MS 1209, and older specifications but I think it was thinner than what was in it as took some out and I looked at some in it and it was more thicker just like what was in the transmission or like a hydraulic fluid ? .So I did drive it afterwards and I never reveved it up and it did not slip so the only thing I can think of is it was thinner and got past the seal and now it found it safe level and quit dripping and I think I can just see it at end if dip stick now ..and when you read the dip stick do you just push the stick down to where it hits or turn it in all the way as its shows different level then as by 1/2' if you thread it in all the way and that makes a big difference on dip stick? Like 1/2 ?