According to Yahoo! Auctions, they will go to the winner's house and "clean, do laundry, cook—we'll do whatever you want for three hours!"
The rest of the auction description reads, "It's okay if you want to do furious cleaning with BiS! It's fine if you want to cook delicious food with BiS! It's okay if you want to fold flappy laundry with BiS! We're waiting for your bid!"
The "flap" pun exists in the original Japanese, too, as the "flappy laundry" is referred to as "shikoshiko sentakumono" (しこしこ洗濯物), with "shikoshiko" also being the Japanese for the masturbatory "fap fap".
Innuendo aside—and BiS seems to be built largely on knowing winks—this band is not going to do anything you want. The auction page clearly notes "no sexual things".
Stunts like this are working for the group. The video for their first single, "My Ixxx", seemed to feature the BiS girls, who are all in their early twenties, completely nude! (They would repeat the trick in the music video for "Ash").
Strategically placed flesh colored tape covered their bodies, and it was no surprise the video got hundreds of thousands of views on YouTube when it debuted last year. It now has over a million views.
Like Momoiro Clover Z, BiS does seem to be having fun with idol and pop culture iconography (note the yanki zuwari above!). Take the group's newest track, Idol Is Dead, which is loud and angry. BiS, however, are not nearly as kid-friendly as Momoiro and have more of a darker edge to them, while retaining the necessary idol "pureness".
So! If you read Japanese and want to bid, you can find the group's Yahoo! Japan auction page below. As of posting, the top bid is well over the equivalent of US$4,000.