Somehow I missed the word waltz in the subject of the original question and went off talking about foxtrot.
Still I think everything still applies.
Keh asked about uses. I originally learnt the hairpin, like most figures, as part of a routine, but I find it useful when I need to take avoiding action. As such I don't have a particular preceding figure but if my LF is free with nowhere to go I might dance a hairpin. Follows could be 4-6 of reverse turn (feather finish in foxtrot)or possibly a reverse corte. If my RF is free I might dance the curved feather and maybe a hesitation change finish or open/closed impetus.
Howard