What ist?

Do the twists follow through or do they go in opposite directions?

Is it some sort of adapter?  If the small end fitted into summat, using the nut in the middle to screw it in to the summat, you'd end up with a bigger, open end - or, in reverse. you'd have a smaller end piece and it would be a reducer.

I know what I mean even if 99.99999999999999% of the rest of the world haven't a clue.

Ir you look down the length from the 'nut' they each twist to the right from either end if that helps?

Sorry veggie, include me in all the nines!