When one synchronizer is shifted into gear the movement of the other shifter forks is blocked by what?

When one synchronizer is shifted into gear, the movement of the other shifter forks is blocked by the design of the transmission's internal mechanisms, specifically the synchronizer's engagement with the gear it is shifting into. This prevents multiple gears from being engaged simultaneously, ensuring smooth operation and preventing damage. The blocking is typically achieved through a combination of detents and interlocks within the transmission system.