I think it works the same way Ryu’s U2 works. You have to land a max range(or very close to it) low forward xx fireball, then FADC into Ultra 2. I’ve done it several times in the training room and that’s the only time it has worked for me. Yes it’s hard and you’re going to need some good reactions because you can’t link E. Ryu’s low forward from any of his normals. You’re just going to have to be able to recognized that you hit low forward from pretty far away and just go for it.
A lot of good players are able to react quickly to stuff like counter hits which hardly gives you any time to recognize that you landed one. If you’re up to the task then go for it. It’s great damage for 2 bars.
EDIT: On 2nd thought, I think it’s possible to combo it off st. MP xx Fireball, FADC, U2. I remember some guys talking about it a while back, never tried it though.
2nd EDIT: Yeah it works, so you could do jab, jab, strong, Hado, FADC, Ultra 2 for about 400 damage. Not every version of this combo works though as it’s already been stated, you have to hit the fireball from nearly max range. Some characters with a big hit box gives you more leniency as you can throw out more jabs and it will always be spaced correctly where as smaller characters like Chun Li it will be harder or maybe even not possible.