Hockey has a few unique attributes compared to every other major North American sport. For starters, it’s played on ice. It’s also an enclosed playing area with no out-of-bounds unless the puck flies above the boards.
But perhaps the biggest difference is that substitutions are made on the fly with players simultaneously skating off the ice and climbing over the boards while the game is going on.
For an outsider, line shifts look like chaos. On the bench, while it happens so frequently that it almost becomes second-nature, it still requires preparation and coordination to prevent unnecessary penalties and avoid getting caught on the break.