There shall not an hand touch it. Rather, "there shall not an hand touch him." The transgressor shall not be seized and apprehended, for that would involve the repetition of the offence by his arrester, who must overpass the "bounds" set by Moses, in order to make the arrest.
Instead of seizing him, they were to kill him with stones or arrows from within the "bounds," and the same was to be done, if any stray beast approached the mountain. When the trumpet soundeth long, they shall come up to the mount.
By translating the same Hebrew phrase differently here and in Exodus 19:12, the A. V. avoids the difficulty which most commentators see in this passage. According to the apparent construction, the people are first told that they may, on no account, ascend the mountain (Exodus 19:12), and then that they may do so, so soon as the trumpet sounds long (Exodus 19:13).
But they do not ascend at that time (Exodus 19:19), nor are they allowed to do so—on the contrary, Moses is charged anew to prevent it (Exodus 19:21-25); nor indeed do the people ever ascend, but only Moses, Aaron, Nadab, Abihu, and the seventy eiders (Exodus 24:1, Exodus 24:2).
What, then, is the permission here given? When we scrutinise the passage closely, we observe that the pronoun "they" is in the Hebrew, emphatic, and, therefore, unlikely to refer to "the people" of Exodus 19:12.
To whom then does it refer? Not, certainly, to "the Elders" of Exodus 19:7, which would be too remote an antecedent, but to those chosen persons who are in the writer's mind, whom God was about to allow to ascend.
Even these were not allowed to go up until summoned by the prolonged blast of the trumpet.