transaxle accent info???

ok my transaxle grt no reverse.all other gears shifts fine.i went to check under the hood of the car.right by theres a air filter right unders there a trans.ok i told my girlfriend to start the car and put it in reverse i saw the cable pulling this round devise its on the top of the trans?any body knows what it call?i put my finger right on the devise and push it to the right and the car kicked in gear start to back up faster.i could not hold it so i let go and the gear went back to netural.and the engine was shacking.what's that round devise that being pulled by the cable ?right under the air filter box.maybe that's why i it cant get reverse.need to know the devise called. thanks any info will help.