Sentence:  Rushing downstream, taking out fathoms of line too freely given, the crafty savage returns on his tracks, swimming deep down in the bed of the stream, and trusting to the weight of the water to <WSD>belly out</WSD> the line and leave the fisherman under the mistaken impression that the intended victim is still pursuing a downward course, the fish eventually brings such a strain upon the line that either it or the rod, or perhaps both, give way ... 
Meaning of the word marked with <WSD> tags:  To cause to bulge or billow outward.