My plank never softens.
Ever...
As for this exercise, I find that isometric work is great for some portion of your training. But most people (including the person in the pic) do it pretty crappily...check your posture, folks. If you can't keep a properly stabilised shoulder girdle and spine (including neck and lower back) then it isn't a good exercise. And certainly not a very good way to train your abs.
Instead, try alternating lifting each leg behind you, fairly quickly without moving the rest of the body or extending the back. Keep breathing as you do...a much better functional exercise...and more fun, too!
Try yoga or pilates...lots more stuff in there to challenge you than this boring non-movement!!! LOL