Question: When the Gemora proposes the idea of saying Lovud and thereby you would have a Lechi How does that work? Lovud is a din, its not something that can be seen. How then could it serve as a Heker? Thanx so much for all you do!!!!
Excellent question! In fact, where a Lechi does not produce a Heker, the Rabanan usually do not allow using such a Lechi (such as ha'Omed me'Elav, 15a, according to some Rishonim, or Korah b'Alakson, 8b). Why should Lavud be allowed?
The Gemara on 9b that you refer to mentions a few cases of Lavud. The one that involves two Chatzeros, one 11 wide and one 10 wide, is a question of separating a small Chatzer that is Parutz b'Milu'o to a large one by creating a "entranceway" between them (Rashi, end of the 9b), and not of making a Lechi as a Heker to separate a Mavoy from Reshus ha'Rabim. In such a situation, a mechanism other than Heker is involved.
(On the top of the page, the Gemara discusses invalidating a Lechi due to Lavud, and not using Lavud to make the Lechi work.)
Of course, if a Lechi is placed within three Tefachim of a Mavoy it works because of Lavud. However, in such a case the Heker is indeed there (i.e., there already is a Lechi with the proper Shiur); it just has to be placed next to the wall of the Mavoy, and Lavud suffices to put it there.