The first time I went scuba diving, a stingray* got between me and my oxygen hose and popped my regulator out of my mouth about 25 feet below the surface of the water. It sounds scary, but all I did was reach my arm back, sweep forward to catch my regulator, put it back in my mouth, and push a button to clear the water out. I didn't even have to think about it. It was muscle memory, because I'd practiced it dozens of times on dry land.
The principle of Practice Where It's Safe is a way to give participants permission to try using the tools and techniques they're learning in the safety of the facilitation, before they go back to the real world. It's a chance to try something out, fail safely, and learn so that when they need it, it's muscle memory.
*Sharks are easier to draw than stingrays. But it was a stingray: