"a noise such as a man might make, involuntarily, feeling the nail go through his hands and into the wood"
Bronzino's Christ on the Cross, 1545
Public DomainBronzino's Christ on the Cross, 1545

This much-discussed paragraph is an allusion to the crucifixion of Christ. Some have suggested that it means Hemingway saw Santiago as a Christ figure. I think it more likely that it is a "mere" allusion, albeit a powerful one, exemplifying the depth of the agony in Santiago's soul. When he sees the two sharks approaching, he knows it is the beginning of the culmination of his struggles.