Tightening your core is actually the opposite of sucking in your ab area. A strong core actually expands the area slightly, adding stability and stiffness to the area. Squeezing your glutes is trying to tighten them in towards your midline and slightly towards the front.
You should be able to feel it if you try to do it while standing. Practice trying to contract until you can feel your butt cheek muscles squeeze in on themselves.
Doing these 2 things should allow you to be very straight in your plank, so sagging and no butt pryamid.
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