Ctrl-s hang terminal emulator?

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http://unix.stackexchange.com – I came across a sentence in vimdoc: Note: CTRL-S does not work on all terminals and might block further input, use CTRL-Q to get going again. and this key indeed hangs my vim. I was thinking that it was the fault of vim, since there was no problem when I use C-s/C-x C-s in emacs nox. However just now when I was reading a manpage and pressed the Ctrl-s, it hangs man as well(I am setting less as the PAGER). So can someone tell me what's happening? The terminal emulators are xterm and lxterminal, and tty also has this problem. And a Ctrl+q puts the process right again in all (HowTos)