This quote comes from Elizabeth Moon.
It's like dancers...my mother once attended a recital that included the son of friends. The son of friends was very proud of his muscles. So when he lifted his partner, he made sure he looked STRONG. My mother said he also managed to make his partner look HEAVY. Weight-lifters can grimace and grunt and drip sweat down their fronts, but male dancers are supposed to make it look easy and big strong ballerinas look like thistledown.
Writers who show the sweat marks (as in forcing readers to endure all their research) are like dancers who emphasize the difficulty of lifts and leaps.
Readers shouldn't see us sweat. They shouldn't see the rehearsal with the missed steps, the staggering, the grimaces and groans. Only the grace, only the beauty, only the music.
(Until they think "Oh, I could write like that, it looks so easy..." Then, THEN, they see the sweat, but it's theirs...heh-heh-heh.)