Never made the connection, but when I was in human anatomy class, we removed the thigh muscle from a frog, hooked it up to a seismograph, and shocked it, to track the spasm.
As the shocks went on, it would eventually cramp up. Putting saline solution (salt water) on it, would release the cramp, and you could do the experiment again.
That's a direct example of salt making a muscle relax. I never thought to connect it to marinating meat.