the error is not in how you hang the underwear (though you should be hanging from the waist for a neat line) but in placing underwear outside at all. outdoor clothes dry outdoors; what is hidden when worn should remain hidden from all prying eyes.
the only time i deviated from this excellent policy was as a lovely, slim young married woman. my older, fatter neighbour would hang her ugly undies on her line where we could not help but see, so I took to putting out my beautiful sets of bras, french knickers and suspenders to annoy her. it did annoy her. and i still find that satisfying, despite now being an older, fatter woman myself. and i keep my (lovely but larger) underwear indoors!