'The Sing-Off': 'On the Rocks' Wants You to Pour Some Sugar on Them ... Yeah! (VIDEO)

Step inside, let's walk this way. Hey, television lover! Baby, let's go all night. 'The Sing-Off' (Mon., 8PM ET on NBC) returned for its fourth episode, and the theme for the show's second round was this -- "Guilty Pleasures." Sweet.

The six remaining groups hit us with some very guilty pleasures indeed, including 'Love Shack' and -- yikes! -- 'I Want It That Way.' But cult favorites 'On the Rocks' continued with their winning ways, busting out an awe-inspiring version of Def Leppard's 'Pour Some Sugar on Me' -- complete with hilarious/"sexy" choreography.

And the best part? That wasn't even their "guilty pleasure" choice. It was their "hard rock" tune. Their actual guilty pleasure choice was ... a very bad 1980s song.

But 'Pour Some Sugar on Me' was the clear highlight of the evening -- with 'OTR's' lead singer collapsing on the stage, then continuing his performance while singing upside-down. (Presumably, this was done to symbolize the whole, uh, pouring-sugar-on-him process.)

Indie-singer and judge Ben Folds admitted that he couldn't hold back a smile while watching 'OTR,' though he asked whether Def Leppard was the best "artistic" choice for the group. After the judging took place, 'On the Rocks' moved on to the next round; and Seattle-based group 'Groove for Thought' was eliminated. 'You Make My Dreams' by Hall & Oates was their song choice ... but in the end, their dreams didn't come true. They were sent home, as the remaining five teams continued in their quest for the final prize -- $100,000 and a national recording contract.