Hull shape

I have heard that a flattish aft section makes a boat go downwind better. Can anyone explain why? In laymans terms?

I’ll take a stab ---- flat underwater sections promote/encouage the boat to plane (rise up and hopefully on top of the hull displacement wave) where there can be a bit more speed. Since many off-shore races are or have long downwind legs, hulls for the round-the-world events are optimized for off-wind sailing. Once up on a plane, hopefully the hull will trave at the true speed of the wind. Boats with less hull weight or displacment (multihulls, skiffs, scows, etc.) need less of the flat large sections, and can often EXCEED the wind in broad reaching legs. (ice boats and land yachts can do same “upwind” in some conditions and the right wind directions - creating their own apparent wind.)

I’m sure others will assist in explanation/theory.

This is what I knows about flat bottom hulls.
I just prepared a little sketch with few words :

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