We are a commune of inquiring, skeptical, politically centrist, capitalist, anglophile, traditionalist New England Yankee humans, humanoids, and animals with many interests beyond and above politics. Each of us has had a high-school education (or GED), but all had ADD so didn't pay attention very well, especially the dogs. Each one of us does "try my best to be just like I am," and none of us enjoys working for others, including for Maggie, from whom we receive neither a nickel nor a dime. Freedom from nags, cranks, government, do-gooders, control-freaks and idiots is all that we ask for.
Planks are isometric core exercises. While many or most exercises require or produce a strong core, planks are pure. They are good to throw into calisthenic circuits. (Crunches are generally not a great idea). Another good core exercise is one-handed kettlebell swings. I also do Roman Chair leg lifts weekly, the back raise thing, one-handed Farmer's Walks, and mountain-climbers.
Contrary to common belief, core exercises do not give you 6-pack abs. 6-pack abs are mainly a product of low body fat.
The key to effective planks is to make them isometrically-active. That means pulling your tummy in and tightening your glutes during the time of the plank. That's a rigid plank, not a relaxed plank.