There is a solenoid in the vacuum advance line. When top gear is engaged, the solenoid turns on and leaks air into the line. So you get zero vacuum advance. Less timing = less performance. But also less emissions. This is the Nissan TCVAS system.

It is an evil system as far as performance and fuel economy go. New cars also reduce timing at highway cruse, but they modify the timing advance electronically which is a much better way.