I would like to divide the question into two parts: 1. Is the rebid forcing in a temorary partnership? 2. What is the most sound treatment for a "natural" system in general? My answer to the first one is that it is forcing. There are drawbacks, but there is less risk that your p will be upset if you end up in a hopless 4♠ after 1♦-1♥-1♠-2♠-4♠, than if you pass down a making game. My opinion on the second one is that I think it is better off as non-forcing, but it heavily depends on what you promise with a 1/1. I am in favour of bidding 1/1 with as little as 0 hcp (especially when using 5-card M) to rescue a vulenerable disaster. However, the probelm with this treatment is that you risk a 2NT respons, showing 18--19 balanced, which is also a disaster. Now, in my current partnership we are evaluating concealing most 18--19 balanced in the 1M (4-card showing) rebids, with the only risk of playing your 5+-card suit (which you always have on a rescue operation like 1m-1x-2NT-3x) on the three level.