I regularly break the six second rule when I'm racing another commander to the station. I drop down to 5 seconds and speed past my competitor and then use gravity braking to slow down enough to drop out of supercruise. It takes some skill to fly close enough to the planet to slow down but not too much. It also takes skill to not drop to 4 seconds unless you can pass two gravity wells on the way to the station.
I do full throttle until 4 sec., then full reverse throttle, pitch down slightly, wait until 3 second, then start pitching up to keep station at about 30 degree up... then 2 sec., and ovetshoot just slightly at about 90 to 100 degree, so speed drop like a rock. There starts the second leg of the spiral. Throttle to middle of blue to keep the tightest spiral until I see station. Then throttle up to 6 seconds, until speed at 1.2M/s, then full throttle to get an early drop.
The thing is, unless you overshoot (station passes the 90 degree plane), the speed won't drop fast. It takes forever to slow down in SC even with full reverse throttle. It's not a classic Newtonian physics. Some witch physics... It's what it is in game. The whole point of the spiral is to minimize the amount of overshoot, but still benefit from the fast reduction of speed with the slightest overshoot.