Have you heard the buzzword for the trendiest new way to train your abs? It’s called “dynamic stabilization.” That sounds complicated, but it’s really just a fancy way of describing an exercise that requires you to hold your core stiff while moving other parts of your body. Think of the plank: In this exercise, you basically get into pushup position, tighten your core–as if you’re about to be punched in the gut–and hold for 30 to 60 seconds. That’s a “stabilization” exercise. Now if you lift one of your feet off the floor and raise your knee to your chest–while maintaining that plank position-you’ve just made it “dynamic” as well. Which brings us to this week’s exercise: It’s called the single-leg jackknife, and it’s one of the toughest dynamic stabilization exercises you can do. Watch the video (left) to learn how to do this hard-core ab move. You might just find it’s the best exercise you aren’t doing.