Or HaChaim on Exodus 19:20/אור החיים על שמות י״ט:כ׳

1 וירד ה׳ על הר סיני, G'd descended on Mount Sinai, etc. According to Sukkah 5 G'd never came closer to earth than ten handbreadths above the Mountain. We therefore have to understand why the Torah uses the expression "on top of the Mountain" when He did not actually descend all the way to the Mountain. Perhaps the Torah wanted to inform us that the שכינה did not descend on the sides of the Mountain although this would have been more than 10 handbreadths above the ground. We had to be told because on other occasions the שכינה descended to other locations on earth which are lower than the top of Mount Sinai. The essential point the sages make is that in keeping with the principle that whereas the Heavens belong to G'd He assigned earth as an exclusive domain for man. We can deduce from the report of the Torah about the wanderings of the Israelites through the desert when the שכינה was present wherever the Israelites made camp, i.e. in places much lower than the top of Mount Sinai, that G'd's presence descends to lower regions above the earth.

2We still need to understand why the Torah did not write: "G'd descended on the top of Mount Sinai?" although we find such an expression immediately following the statement that He descended on the Mountain;" Perhaps G'd thought that if He described the descent of the שכינה in these words they could be misunderstood. In order to avoid misunderstandings, G'd is first described as descending on the Mountain, i.e. to within ten handbreadths of the top, whereas later on the exact part of the Mountain G'd's presence descended on is described as "the top of the Mountain."

3Another version of what happened is expressed in the Mechilta according to which G'd lowered the upper heavens on to the top of the Mountain. According to this explanation G'd Himself did not descend at all. The word על would refer only to the fact that G'd's position on top of these upper heavens would be lower than it had been previously when speaking in terms of the distance of the heavens from earth.

4Another meaning of the verse is that as G'd began to lower His presence onto the Mountain, the Mountain began to shake and rise towards G'd much as a servant runs toward his master when the master approaches. The Torah here did not really tell us to what place exactly G'd descended; the verse was more concerned with informing us that though the Mountain is basically inert matter, in this instance it was transformed into a living creature so that it could rise before G'd would descend to what used to be its top. Afterwards the Torah describes the target of G'd's descent as "the top of the Mountain." The major message of the verse is that the Mountain rose towards G'd before G'd had reached the top of the Mountain during His descent.