Tuppence turn: thanks to Lalwendė for supplying the text...
Quote:
The Balrog made no answer. The fire in it seemed to die, but the darkness grew. It stepped forward slowly on to the bridge, and suddenly it drew itself up to a great height, and its wings were spread from wall to wall; but still Gandalf could be seen, glimmering in the gloom; he seemed small, and altogether alone: grey and bent, like a wizened tree before the onset of a storm.
From out of the shadow a red sword leaped flaming.
Glamdring glittered white in answer.
There was a ringing clash and a stab of white fire. The Balrog fell back and its sword flew up in molten fragments. The wizard swayed on the bridge, stepped back a pace, and then again stood still.
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At this moment, the two have had a major power encounter: enough to (a) shatter the sword of the balrog and and make him step back, and (b) make Gandalf sway on the bridge. So the Balrog had already taken a 'hit'. The second dose of power (from Gandalf's striking the bridge) may have affected him as well, but either way, the first hit may have weakened him enough to make him unsteady.
And meanwhile, wasn't the air in the mines, especially at the bridge, cold and sinking? No hot updrafts? So even a jet (or an eagle) might have dropped in those conditions, as they sometimes do when encountering downdrafts while crossing mountains.