Three minutes on the rower followed by quickly hopping on the ground to hold one minute of plank - that was part of my CrossFit workout today. Four rounds of one-minute plank holds is no joke, but it wasn't until Champlain Valley Level 1 CrossFit Coach Laura Matuszak gave this verbal cue that I wasn't really engaging my abs.
Although you can definitely focus your attention on your upper body by really pressing into the floor and keeping the shoulders stacked over the wrists, plank is supposed to be a core-strengthening exercise. She said that you need to actively pull your belly in toward your spine, rounding it up slightly, similar to how you'd draw your navel in when doing a hollow hold. This will help prevent a sagging butt, which not only doesn't work your core but can put painful pressure on your lower back.
Think about coming into a plank position (either on the hands or the elbows), sticking your butt into the air slightly, then tucking your tailbone in to engage your abs. This will help to create a slight pelvic tilt. Note that your back isn't arched up toward the ceiling in a C shape; your spine is still in a neutral position. You'll know you're doing it right because you will instantly feel your abs working (and your body might even shake!), and you'll feel your muscles burning way more after a 60-second hold - I definitely felt the difference.