You may remember Jennifer Garner looking pretty spectacular ( best dressed list-worthy, really) in a custom, half-off-the-shoulder, half-beaded Atelier Versace gown at this year's Oscars. Well, turns out, the show-stopping dress wasn't all that glamorous — and Garner was in a lot of pain that evening.The actress went on The Tonight Show with Jimmy Fallon and revealed what was going on underneath the dress. First, she explained to Fallon, and those otherwise unacquainted with red-carpet dressing, exactly how a fancy, custom awards-show gown feels."What they do is they move your organs around," Garner joked. Two people helped secure her into a metal corset (compressing her ribs in the process), she described. Fallon, trying to understand this unfamiliar experience, asks if it's similar to Spanx — to which Garner replies: "No, dude. It's not like that. It's intense."How intense, you ask? "There's metal inside [the dress], and these people move your ribs, move your liver out to the side, and they pop you in to keep it in place," Garner told him."That can't be too comfortable," Fallon jokes. Uncomfortable would be an understatement."I had a panic attack in my ribs," Garner recalls. "Everything [in my body] started to freak out." But she was doubly trapped — both by her custom creation and by the live broadcast. So, she waited for a "blessed" commercial break to make her way to the safety of some off-camera space backstage.To make matters worse: Before you even get to the corset, she warns, there's a thong bodysuit you need to slip into. Now, if you're thinking that would lead to some awkward bathroom breaks, you're correct: Garner called upon her close friend and Oscars date to facilitate the trip, which involved skirt-lifting and whatnot — only to find that the hook-and-eye opening of the bodysuit was in "an intimate position," and she would need to get it out.Garner was quite a trooper, making it through the ceremony and the after-parties without an outfit change. "I just feel like you should know the truth about these things," she told Fallon of the struggles. Believe it or not, her anecdote isn't all that unique — we've learned a lot about award-show shapewear lately — but watch her tell it in the video, below.