I don't see it mentioned here.
I think I saw some pros do it once or twice. The idea is to do pylon then immediately expand before Gateway, utilizing the Mothership Core to defend against early pressure.
If successful and you defend against early pressure, you gain a speed boost vs other openings such as Gateway expand, and spare the resources required by Forge+cannons which are mostly useless on offence.
Is it a sustainable build? What are some tips on pulling it off correctly? Vs which opponents?