In the "Power Bowling" training level, when you get to the 91 pin level, make the ball ride the top of the blue boundary wall all the way to the end. After that, you should hear a click, then the sound of an explosion. Shortly after that you will see all 91 pins drop, giving you a strike.
There is a secret button that destroys all standing pins is in Training mode called "Power Throws". If you ride the rail you can get this button, making an explosion and knocking down all the pins. Note: This only works on the 91 pin part. The button is red and visible when you press Up.