"put his hand back into Nebuchadnezzar’s furnace so quickly"

An allusion to the book of Daniel in the Bible. Daniel interprets various stories and apocalyptic visions. In one of these his companions Shadrach, Meshach and Abednego are cast into a fiery furnace by Nebuchadnezzar but come out unscathed; in another, a disembodied hand writes a prophecy of calamity on the wall at Belshazzar’s feast – mene, mene, tekel u-pharsin.

The reference in the book implies that having escaped without damage, David would be inviting doom to attempt the same crossing again.