A discussion on BBF underlined yet again a concept that I believe to make sense.
The origin to the idea was a combination of a discussion with Ken Eichenbaum in Philadelphia concerning a specific "exception" where one might suppress a 4-card spade suit after a 1H opening, coupled with a Bridge Bulletin hand with the same principle. The idea is that with a light hand, a practical response to a 1H opening with four spades and six clubs is occasionally 1NT, planning to bid clubs later. (For the skeptics, the hand in question in the expert forum was handled frequently with that approach). This suggested that a "impossible two spades" is no so impossible:
2D-P-2S = weak with four spades, six clubs?
This could be tweaked somewhat to include 3136 hands. This caters well to the hand where Opener (not playing Flannery) has 4531 shape and finds himself "forced" to rebid 2D, disliking this, of course.
The first question, then, is whether Responder or Opener should have the fourth spade (to bid "impossible 2S" or to pass the same). It seems that the hands where Responder bypasses a 4-card major are less frequent than the 3-card options for Responder, such that Opener should be 4-card to pass.
This, then, leads to the BBF problem. Opener has a trashy hand with 4630 and lousy (9-high) hearts.With the "not impossible two spades" approach, 2D comes to mind, as it seems to increase our chances of landing well (you could end up in 2S for a fine result, and 2D therefore is less frequently passed when that is bad). One concern was propelling the auction when Opener has 5-5 reds, but it still seems that you end up OK when and if Responder hits Opener with the "not impossible two spades" response.