Speed-drive makes the chainring spinning at a higher rate. This means, that a 38 tooth chainring, if speed-drive is engaged, works like a 1.65 times larger one (63 tooth chainring). If you have a 44 tooth chainring, with speed-drive engaged this chainring would equal a huge 73 tooth chainring.

That's why speed-drive is often used for fast bikes like recumbents, or folding bikes, where it compensates for the small rear wheel.
Per piece: € 690,96