"Dancing With the Stars" went unplugged last night, but the looks for the show's first-ever "acoustic night" still needed to be electric.

So its Emmy-winning costume designer, Randall Christensen, amped up the wardrobe to offset the pared-down decibels -- which explains why this was the week "Jersey Shore" star Mike "The Situation" Sorrentino finally flashed his famous abs.

StyleList chatted with Christensen about our favorite week-4 costume designs as well as some of his own. Come along as we hit the dance floor.

Bristol Palin and Mark Ballas: When StyleList saw 19-year-old Palin's belted, long white shirt, we figured she had a change of heart about her threat to go sexier this week. Turns out the costume was her version of sexy! "Mark and I had wanted to do a dreamy chiffon, but the music didn't lend itself to the look," Christensen says.

Instead, the oversized shirt draped over a black sequined sheath channeled sexy in a new way. "You don't have to bare it all to prove a point," says the designer, adding that the super-short costume accented Palin's "beautiful legs."

Florence Henderson and Corky Ballas: Mrs. Brady's "get-a-room" rumba made judge Carrie Ann Inaba blush. Christensen says his muse and her sexy dance inspired him to pull out a $300-a-yard, textured metallic European fabric he'd been saving for the "right person."

"She's proving that a woman at any age can be fun, flirty, and yes, sexy," he says. The shorter, more fluted drape of the dress "was not your typical rumba costume, but a great contrast to the other costumes." After an Oct. 8 fitting, Christensen added slits on the side and more fullness, which gave the dress a "sassy, flippy" quality. "Awesome, wasn't she?" he asks. We thought so!

Sorrentino and Karina Smirnoff: Christensen once told us that flashing the reality star's abs might be "too predictable." But, "It's week 4. C'mon don't you think it was time for the real Situation to be revealed," says Christensen of the reality star's split-open bright red shirt.

Jennifer Grey and Derek Hough: It was perfect 10s for Grey and her black lace dress over nude mesh, which was Christensen's (and StyleList's) favorite look of the night. "Argentine tango screams black lace and skin!" he says.

Grey's decision to drop fishnet stockings from her costume and instead show tanned legs with nude heels was supported by the designer, who says it enhanced the look. "Jennifer was smoking hot... and I'm still reeling from the performance." The slight flounce at the hemline (included after her Oct. 8 fitting) "only added to the intense footwork and flicks she did."

Tune into "Dancing With the Stars" tonight at 9/8 c on ABC to see who will be next to go.

For more behind-the-seams "DWTS" looks, read last week's costume recap.