According to Leggero and a shocking video obtained by TMZ, here's how the X-rated show went down:

Natasha hit the stage at the Improv Wednesday night after funnyman Bert Kreischer crushed it with his first performance of the night, which included ripping off his shirt and telling jokes naked from the waist up. The audience loved it, eating up Bert's performance.

So, Leggero came up with the idea to copy Bert's bare-chested routine since he was such a hard act to follow. She put it this way, "If the boys can do it, why can't the girls?" Good point, Natasha!

But, Leggero kept her plan a secret because she wanted to go with her gut and didn't want anyone to try to talk her out of it.

After her friend Bert killed it on stage a second time, he introduced Natasha for her next go-around, as she paraded out in her overalls with her top on.

Now, this is where things got really interesting as the camera was turned on and started recording.

Check it out ... Natasha is doing a sexy dance in front of everyone as they are completely oblivious to what's about to happen. She first removes her jacket, then the straps to her overalls and then her shirt, completely exposing her bare breasts.

The whole place goes NUTS, especially the dudes in the audience. In fact, one guy screams, "Holy s—t!!"

Leggero throws back on her jacket to cover her boobs before diving into her comedy act as the clip cuts off.

Natasha's funny peep show was part of Skyler Stone's Comedy Rocks event at the Improv, which also featured several other A-list comedians, such as David Spade and Tiffany Haddish.