Does not move with wind:[1] A wall is only defined as a wall if it is unable to move with a common wind. If a common wind can blow the wall, as is the case with sheet walls, then it is invalid. [If the wind can move it even slightly back and forth then it is invalid, even if the wind is unable to make it fall completely to the ground.[2] Some Poskim[3] interpret this to mean as follows: If the common wind can blow the sheet walls a distance of three Tefachim in a way that it invalidates the wall, then it is invalid. For example, if it can blow it three Tefachim upwards from the ground, or three Tefachim away from the Sechach, or that it creates a stomach of three Tefachim deep in a way that it does not contain ten Tefachim of wall within three Tefachim of width, then it is invalid even when the wind does not blow.]
[1] Michaber 630:10; M”A 630:16; Elya Raba 630:16; M”B 630:48; See also: Admur 315:3; M”A 315:3; Chavos Yair 184; Beir Heiytiv 240:19; Siddur Yaavetz Mosach Hashabbos Mitos Kesef 7 Chulya 3:8; Levushei Serud 315; Ashel Avraham Butchach Tinyana 240:11; M”B 240:25 and 30 and Biur Halacha 240:6 “Ad”; Taharas Yisrael 240:44; Kitzur SHU”A 150:6; Kaf Hachaim 240:44 and 630:65; See Piskeiy Teshuvos 240:17; Sheyikadesh Atzmo 32 footnote 11
[2] M”B 630:48
[3] Chazon Ish 77:6; Piskeiy Teshuvos 630:9