Often used at the end of a sentence, they can actually be used anywhere at all:
e.g. "There is no way we can reach the summit before nightfall. This blizzard will prevent us seeing where we're going and we're only half way up!" can be reduced to:
"There is no way we can reach the summit before nightfall ... we're only half way up!"
The message is conveyed clearly, but be careful not to change meaning or you could be accused of mis-quoting.