Towards the end of Vidui (על חטא section) there is a list of sins that are mentioned by punishment, instead of by type. Some people strike their chests while mentioning these sins, and some do not. Should one strike one's chest here, and if so, at what word - חטא, חייבים, שאנו, the mention of the punishment (in which case, does that mean that some lines get more than one strike?), etc.?
The Chabad custom is to strike one's chest when saying the word "חיבים" in the last section of the Al Chet. It is mentioned in Sefer HaSichos of the Friediker Rebbe 5705 page 9 (my own translation):
The Chosid R' Aba Person once asked the Rebbe Maharash how many times one strikes his chest on Yom Kippur. From the number he answered, R' Aba Person understood that we also strike by "על חטאים" and by "סלח לנו מחל לנו".
Striking specifically by the word "חיבים" is brought in the Kehos Machzor with customs at the bottom.
See Nitey Gavriel (Yom Kippur Perek 34 Ois 13) who cites this custom in the name of the Mekor Chaim.