Cross-Country Skiing, by Ned Gillette and John Dostal
(Mountaineers, Second Edition 1983), page 35:
"Get an earlier and more powerful kick by starting to put
pressure on your skis with your toes; imagine that your boots are
cleated and that you're trying to get those cleats dug right into the
snow."
"toe-press versus ball-press" thread, April 2002 on
rec.skiing.nordic: