If you're actually interested
It's a module that connects to the gearbox containing a second clutch and a rotary gear that effectively steps up the gear ratios by a third, leading to a higher road speed relative to the engine.

So BK was essentially correct.
( Kroney, Mon 20 Jun 2011, 15:12, 2 replies, latest was 15 years ago)

*pulls shirt over head*
*runs around room*
( Boss Keloid Got your mother's maiden name tattooed on my arm, Mon 20 Jun 2011, 15:13, Reply)

I thought that it meant that you couldn't reach your highest speed
but was generally more efficient for higher speeds
( Vipros. clever got me this far, then tricky got me in, Mon 20 Jun 2011, 15:13, Reply)

Well it would be more efficient for higher speeds
Because by maintaining 70mph, you could drop your engine speed by a third. It also means that if you engage it at 70mph and maintain engine speed, your road speed would be a third faster.

Edit: None of this explains why they didn't just chuck a fifth cog in the box like they do now.
( Kroney, Mon 20 Jun 2011, 15:16, Reply)

I despute that the function of it is to make you go faster

( Vipros. clever got me this far, then tricky got me in, Mon 20 Jun 2011, 15:18, Reply)

Nevertheless, that is its effect.

( Kroney, Mon 20 Jun 2011, 15:22, Reply)

that wasn't the question though

( Vipros. clever got me this far, then tricky got me in, Mon 20 Jun 2011, 15:49, Reply)