The oscillating saddle is highly recommended if you plan to mow a lot of grass or brush on undulating surfaces, because it allows the flail mower to follow the ground contours MUCH better. The best way of visualizing the benefits of the oscillating saddle can be seen in the YouTube video below (specifically around 1.15 minutes into the video).
The oscillating saddle is basically a heavy duty steel parallel linkage, with grease fittings to minimize wear over time. The oscillating saddle can be added to all TLBE flail mowers. If you plan to shred a lot of small trees (down from the top) the oscillating saddle can be locked into place with a pin.
Please note the oscillating saddle is not recommended on mini excavators weighing less than around 4000 lb as it adds an additional 88 lb to the flail mower, which could result in stability issues when mowing (especially with the arm all the way out).