Using your mouse cursor, left click on the pin you want to remove.

You can click the pin anywhere you want, not just the pinhead. This will instantly remove it.

Once you pull a pin, it will instantly have an impact on one of your balls.

You generally need to move the blue ball toward the pink ball.

Depending on the level, you can also pull a pin to move the pink ball.

Some levels will require you to pull the pin at the right time to dodge traps and enemy balls.