Why did Hashem tell him not to come upon them?
'Rashi', Malbim (Shmuel II, 5:23), from Shochar Tov 27:2: Sha'ul lost his kingship because he did not wait for Shmuel. David is being tested ? he must wait for Hashem, and not rely on his own strength!
Radak (Shmuel II, 5:23): It is not yet the time for the salvation.
Radak (Shmuel II, 5:24, citing Shochar Tov 27): You have no permission to strike them, even if they are close to you, until you see the tops of the trees moving.
Malbim: Ezra explained that "Lo Sa'aleh" in Shmuel II, 5:23 means "do not come upon them from the back", i.e. like the first time. Rather, go to the side. (Malbim ibid. ? you will be on one side and the trees on the other side. Refer to 5:24:3:4.)
What is "mi'Mul ha'Becha'im"?
Rashi (Shmuel II, 5:24 citing Targum Yonasan: It is opposite the trees. Radak ? they are Tusim (berry bushes). Radak (ibid.) - the same applies to "Emek ha'Bacha" (Tehilim 84:7).
