The "Miracles From Heaven" star donned a black one-shoulder Atelier Versace gown to the 88th Academy Awards last month that created the illusion of a waistline for the mother of three in a "legit" way.
"What they do is they move your organs around," Garner, 43, told host
And it's nothing like Spanx, which Fallon thought might be the case.
"No, dude, it's not like that. It's so intense," she said. "It's not like Scarlett O'Hara. There's metal inside it and these people just kind of move your ribs and they move your liver to the side and they pop you in."
If that doesn't sound comfy, it's not.
"It turns out four hours in it's not [comfortable], but to keep you in place you step into a bodysuit first," she said, describing the suit she wore as "the bottom of a leotard, but like a thong."
Later that evening, the Golden Globe winner said she "had a panic attack" in her ribs. "They were spasming. I was having a muscle spasm. Everything in here started to freak right out," she said.
Garner said she couldn't breathe and that she had to stand up, even if people will "think you're Kanye West."
Mercifully, a commercial break gave her a minute to get backstage. But that's when she realized she had to go the bathroom and a friend had to assist her, bridesmaids-to-brides style.
"So we go into the bathroom because she's going to have to hold something up. It's not pretty," she said to Fallon's chagrin. "I just feel like you should know the truth about these things. This would not happen [to a guy].
"It's worse than a snowsuit. It's worse than a toddler in a snowsuit, because of the bodysuit and the connection of the bodysuit was in an intimate position," she said, motioning that the bodysuit's hook-and-eye closure was pretty much in her lady parts and she couldn't reach it.
But her good friend helped her save the day -- and her ribs.
Ah, the things people do for fashion.