Made famous by Rocky Balboa (and Mr T) knee to elbow crunches are one of those conditioning exercises you use to hit the "badass training" button.   

What's involved: Abs, lateral abs, core, intercostal muscles, lower abs, front hip flexors.

Powered by: Lateral abs (mainly), abs.

Tip: Exhale sharply as you twist to flatten your lower abdominal wall and for an extra twist, try taking the outside of your elbow to the outside of your knee, each time.