It's not the spring that will burn up the transmission, it's lack of throttle pressure.
The slotted rod is the throttle pressure linkage. As the throttle starts to open (go rearward), the throttle pressure linkage should begin to "push" the throttle pressure lever at the transmission to move. The more throttle you give it, the more the throttle pressure lever should move, thus applying more throttle pressure inside the transmission. Throttle pressure tells the transmission when to up shift (along with the governor), when to downshift and if you have Part Throttle Kickdown, it tells the transmission when to downshift into "passing gear". Driving without any throttle pressure will burn up a 904.
Highly recommend picking up a 904 transmission book. Lots of great information. You can also use the search function to search FABO for throttle pressure info.