"For the person going backwards, which is in the main the lady in the Foxtrot."
Once out of medalist the man probably goes backwards as much as the lady, but anyway...
"We all know that the suporting foot does not lower until the moving foot is under the body."
No, this is far too late. It is important that the arriving foot lower with control, however it must be down with the weight in the heel before the moving foot draws under the body, because the instant the moving foot is under the body the toe must begin to release. Actually, if you do it right, the body will already be past the standing foot at the time the feet pass, so by the time the moving foot is under the body the toe release will be perhaps visibly developed.
"As we all know premature lowering causes the front knee to straighten the toe to pop up and will cause a falling away from your partner. Not my words, but those of the 1986 British Amatuer Champion."
Premature lowering yes, but this is a beginners problem. Many top level dancers today can be seen to delay the lowering far too long, with the result that they never accomplish the heel push phase of departing the standing foot - by the time their heel is down, their weight is already arriving in the other foot. Such a loss of grace...