Here's a crude drawing of my flail mower action. There's four segments on a central shaft. Each segment has eight flails. Each flail is offset about 2" from the one in front of it. This means that each flail is making a fresh cut. If you look at the picture above, you'll notice that the cutter is over-driven. I forget the exact size of the pulleys but I recall it being about 2.9:1. At 540 pto speed, that puts the cutter shaft turning around 1566 rpm. When it's cutting, it sounds like a buzzing noise.
I have both cutters too. The flail will make a cleaner cut and it doesn't have a problem with 3-4' green Johnson grass or 4' dried Blood Weed. So "tall grass" is relative to the viewer. It makes short work of small brush up to about 1" and it doesn't throw chips all over the place. (Great for those seedling Mesquites around here.)
Both have a place around here but after one use of a rotary, the flail mower gets all the maintenance duty. And if we ever get any rain, I may use it again--This year!