Internal geared hubs?

that's a reasonable guess.

Once you're out on the road the hub spins up to speed well – even with three extra gears, it's going to be about 100g lighter than the eight speed at about 1590g, that's 120g lighter than a Speedhub
lighter than a Rohloff? that'll win it for me.

from here, the more I read it, the more fantastic it sound.