Elbow side plank twist

Could someone please explain to me a few key points about this movement, what's exactly meant by do not sag or arch during the twist?

Thank you

Unfortunately it says I don't have permission to see it

I can't access the F1 wiki either... Unfortunately i haven't been able to access ever since the new forum update

regarding your question, here is a few key points:

 The key thing to be aware of in this excersise is the height of your hips relative to the rest of your body. Notice how the athlete in the course video have the hips in line with his head when he's at the bottom of the rep. The hips need to stay in line and not lift above or fall behind the line during the twist. Think of this as a straight body element, not arched, not hollow. If there is a tendency of wanting to hold the hips too high, this will lead to a bit of side arch, this will inevitably lead to piking.  

Another common mistake when browsing the forum would be to rely too much on using the elbows to get to the ground. Leading with the elbow will shorten the ROM. Therefore,  keep the elbow retracted as much as possible until you no longer can.

All this was explained very well in the F1 course wiki with pictures, wish it was possible for you to see it