Finesse means avoiding the temptation to dig your arm deep into the water and muscle yourself across the pool. In swimming, finesse means using your brain, not your brawn.
Swimming fast is a skill that demands great strength and stamina. Yet swimming is neither baseball nor boxing. One cannot hit the water like a ball crushed over the center field wall or knock it out in the first round. One cannot simply power through the water. To swim fast, one also needs great timing and swimming finesse.
What does finesse mean with respect to swimming fast? In water, where frontal drag forces are so compelling, swimming finesse means learning to swim with the lowest possible drag forces. It means pulling with an arm motion that may seem totally inept or awkward, yet works better.