No hand may touch it, for [if one does] he will be stoned and cast down. Whether animal or human, he will not live. When the ram’s horn sounds a long blast, they may [again] ascend the mountain.
Exodus 19:13
The shofar blast indicated the withdrawal of G-d’s presence and the end of its sanctifying effect on the mountain.
The open revelation of G-d’s presence on the mountain temporarily suspended the people’s free choice since they could not deny G-d’s existence or concern. Once the Divine revelation ended, there was room again for doubt or even rebellion. Thus, the blast signaled the return of free choice and independence of action.
It is precisely because of this independence that our accomplishments count. If there were no alternative to devotion to G-d, it would have little significance. When there is no motivation from heaven, we must muster our own initiative.
The sanctity that G-d infused into the physical world at the Giving of the Torah was superimposed and therefore fleeting. In contrast, the sanctity that we infuse into the world comes from within the world itself, and is therefore permanent. Moreover, G-d created the world for this very purpose.
G-d led the people with a pillar of cloud by day and a pillar of fire by night. As the Egyptians caught up with the people at nightfall, the pillar of cloud hid the light of the pillar of fire from the Egyptians, leaving them in darkness and the Jewish people in the light.
-- Daily Wisdom Volume 3