What is the "gleam in the gloom" and how does it turn the tide of the battle?

Exactly when overcome for the powers of good appears to be unavoidable, Bilbo's sharp eye notices the hawks drawing nearer. The sparkle alludes to the last beams of sun and the agony of the losing fight as the falcons swoop down to complete the war unequivocally. This was taken from the book. The Hobbit page 268,

Where’s the selection?