Pia Toscano dancing with the stars Mark Ballas american idol PToscano042611_02_X17460WM.jpg

Pia Toscano belted out "I'll Stand By You" on Dancing With the Stars last night, and backstage, boyfriend Mark Ballas gave her a hand ... on her rear. Horizontal mambo, anyone?

The sexy songbird nailed her DWTS performance, and no doubt, she was pumped to see Mr. Ballas dancing while she performed on stage.

See Pia's incredible performance ... after the jump.