How does Dr. McCausland react to the “miracle”? Explain the “clenched Hands”?

I don't think the doctor believed that Willie was in fact dead to begin with.

“But clenched hands?” said Dr McCausland. “Yes,” said I, but did that mean that Willie could not have been dead? “Obviously he was not dead,” said the doctor; “if he had been dead I would not have been talking to him a few minutes ago. I think you may safely leave it to me to say when people are dead, Dunny,” he continued, with what I am sure he meant as a kindly smile. It had been a strong convulsion, he told my parents; the tight clenching of the hands was a part of it, and an unskilled person could not be expected to detect very faint breathing, or heartbeat either. He was all reason, all reassurance, and the next day he came early and did an operation on Willie called “tapping”; he dug a hollow needle into his side and drew off an astonishing quantity of bloody urine. In a week Willie was up and about; in four months he had somehow lied his way into the Canadian Army; in 1916 he was one of those who disappeared forever in the mud at St Eloi.