Sorry if this is common knowledge. I tried searching but the only answers I could find were the same school yard theories from nearly 30 years ago.
It seems a lot of people have their own strategy for breaking the glass, whether it's using Al Macinnis or letting one rip from the blue line.
The truth is a whole lot simpler than that.
I found an interview with NHL '93 programmer Jim Simmons from Inside EA Sports (Volume 2 Issue 1, 1993). In it they ask what's the secret to breaking the glass.
"I've built a small sweet spot into the glass panel directly behind the right half of the goal - it's only three pixels wide by five pixels tall. That's about the size of the puck. You have to hit that sweet spot right on, extremely hard. Using a player that has a really hard slapshot helps, but luck is usually the strongest determining factor in getting it to shatter."
I look up whatever videos are available of breaking the glass in NHL '93/94 and sure enough, it's always in the same spot. Shot speed is not a factor, but it helps increase your odds of firing a wild puck at the "sweet spot".
The spot is roughly in between the feet of one of the fans. Anytime you hit it, the glass will break regardless.