How do I shoot threw spawn doors with pyro?!?!

[Larry]

I have recently seen people shooting threw the spawn doors as pyro and I can not fiqure out how they do it. I'm frustrating me because I keep dying to it. Could someone please tell me how to do it because I want to know if there is a certain distance that I have to stand to not get hit by the flames. Well also because I want to try and kill someone with it anyways that's not the point. If someone can tell me how to do it that would be greatly appreciated!.

[Marvash Magalli]