There is a brake on the side of the gearbox
Note it is not on the wheels it is on the gearbox layshaft
It locks the diff same as the P position on an auto box so the wheels can only turn in opposite directions
The brake is on the clutch pedal and works when it is fully depressed
When you take your foot off the pedal the lever should return to the neutral position
There is a spring that does that
If the pedal returns as it should then the transmission is on the way out
Quick & nasty test is to try & push the mower , engine off & brake off
The wheels should lock up when you try to push it
If not then the tranny is getting very worn