from Kevin Woodley at NHL.com,
In the simplest terms, narrowing the stance improves a goalie's mobility by keeping his feet underneath him and giving him more leg extension with which to push. Conversely, if a goalie is already spread out too wide with his legs, there isn't much extension left to make a powerful, controlled push, whether it's left to right or up and down the crease.
"It's just easier to move," said Detroit Red Wings goalie Jimmy Howard, who has a .915 save percentage in 2 1/2 seasons since making a similar stance adjustment after 2015-16, when he had a .