Bill Kaulitz, the younger of the Kaulitz twins, usually sings all the vocals, except during live performances when bassist, Georg Listing, backs-up. Also, during the song Raise Your Hands (German: Wo Sinde Eure Hande), guitarist-and-elder-twin, Tom Kaulitz, joins Georg and Bill in the chorus.
However, the Tom chorus is usually a reference to Tom's back-up vocals in track two (Monsoon) of the band's English Album: Scream. Tom sings a harmony during the last chorus of the song, creating a bit of low-voiced chaos behind Bill's vocals.
Monsoon can, of course, be found on YouTube, iTunes, and almost any other music site, given that it is one of Tokio Hotel's top English hits. The German version, however, does not have this back-up vocal effect, and, there is wide-spread controversy between TH fans about whether or not the vocals really are those of sexy hexy Tom, instead of a voice-over by Bill.
Most Tom-fans are fairly sure it's him singing, and have even edited out Bill's vocals before posting minute-long clips of the chorus on various websites. The low, guttural vocals do sound a lot more like Tom than Bill's usually clean (both singing and speaking), clipped tone. There are even versions where fans have edited out the crescendo of the instruments as well.
Note: One might say that the chorus in Raise Your Hands could also be called a Tom chorus, given that he sings in it. However, the explosion of noise created by the harmony in Monsoon, which is probably supposed to remind the listener of a monsoon, is much more noticeable than that of RYH.
Santi: "What are you listening to?"
Lexi: "Tokio Hotel, of course! I'm listening to the Tom chorus."
Santi: "The what?"
Lexi: "You know, where Tom is singing the chorus of Monsoon? Someone edited the original song by ramping up Tom's vocals and then they posted it on youtube. Love it."
Lotus: "I'm sure Santi doesn't want to know."
Lexi: "BUT GUYS! Tom's so sexii hexii! I'm like, irrevocably addicted to the Tom chorus!"