Early the following year, K. S. Srinivasan, a student at Madras Christian College who'd known Ramanujan back in Kumbakonam, dropped by to see him at Summer House. "Ramanujan," he said, "they call you a genius."
Hardly a genius, replied Ramanujan, "Look at my elbow. That will tell
you the story." It was rough, dirty, and black. Working from his large
slate, he found the quick flip between writing hand and erasing elbow a
lot faster, when he was caught up in the throes of his work, than reaching
for a rag. "My elbow is making a genius of me," he said.
Why, Srinivasan asked, didn't he just use paper? Can't afford it, replied Ramanujan. He was getting money from Ramachandra Rao. But that only went so far. Paper? He'd need four reams of it a month.
From Robert Kanigel's The Man Who Knew Infinity [0].
My understanding is that Ramanujan did most of his derivations on slate and tended to preserve only the results on paper. His insight was tremendous, but it was backed up by a tremendous amount of elbow work.
[0] https://en.wikipedia.org/wiki/The_Man_Who_Knew_Infinity_(boo...