How does this work??

I'm a total beginner, and i just bought st21, but i just don't get this. How can you use Don!! cards on you're opponents turn??

And does K.O mean just by a attack or also by a effect?

Thank you!

I'm a total beginner, and i just bought st21, but i just don't get this. How can you use Don!! cards on you're opponents turn??

And does K.O mean just by a attack or also by a effect?

Thank you!