In the song, it sounds like Danny is singing "repelling down from above", but the line is actually "rappelling down from above", since Isabella and the other girls descend on ropes during the song, completing the whole ninja metaphor. This has also been confirmed by Swampy Marsh.
During the dialog between Vanessa and Johnny, the crowd and Danny can be heard singing a new part of the song faintly in the background. In the Phineas and Ferb soundtrack, it can be heard that Danny says "Yeah!, You Snuck Your Way Into My Heart!"
In the episode version when Danny sings, it appears that he has more singers backing him up, but Sherman and Bobbi's mouthes are closed. In the album, this was edited so that only Danny's voice is heard and no other.