In the "Power Throws" training game for bowling, you might notice 2 red buttons at the end of the alley - 1 left and 1 right. When you get to the final bowl for 91 pins, you can bowl the ball along the top of the barrier on either side and hit this button.

Move your Mii all the way to the left or right, and turn the aim 2 or 3 clicks towards the barrier. Let go of the ball at the highest point possible, with just a bit of spin to keep the ball on the barrier.

If successful, you will hear a click, the screen will shake and all the pins will fall down.