A couple of observations/questions from seeing the website:
1. Nothing is said/shown how to get the hitch off the mower. The quick-disconnect with the splined shaft is shown, but the hitch that goes from the disconnect to the mower is not shown. If it is disconnected, the dongle remains on the mower, which is probably undesirable.
2. Nothing is said about reversing.
3. Nothing is said about being able to make full Z-turns. It appears not to be the case. the wheel assembly will not rotate under the hitch -- at least from what I can see from the pictures. This situation is also part of the reversing question (2) above. A single-wheeled Velke is designed so that the wheel will rotate a complete 360 under the hitch. This enables reversing, with the wheel being pulled backward, and also permits the true Z-turn.
4. It is not entirely clear how the spring works. The words speak about shock between the mower and the sulky. This shock is trivial over against the shock of wheels riding over uneven terrain. The large wheels do look nice, however. But, I wonder the value of the added part of the spring.
For those discussing being "behind the controls," the distinction is between a trailer sulky, and a castered sulky. A trailer has a pin connection, enabling it to rotate about a vertical axis. The wheels on the sulky remain fixed in the plane normal with the axle running across the sulky. The picture with the operator makes it clear that this unit is a trailer sulky.
The castered sulky is firmly fixed to the mower, making the sulky and mower move as unit. There is no pivot point about a vertical like the trailer sulky. The sulky wheels turn on a spindle, each wheel independent. They rotate on a vertical axis, with the wheel itself free to rotate about the spindle a full 360 degrees (unlike the trailer, where the wheel remains at 0 degrees). The castered sulky remains behind the mower, meaning the operator remains "behind the controls" (I believe that was the phrase used earlier).
I have a Bullrider convertible. No, there is no soft top -- not that kind of convertible! Rather, with placement of pins, it can be made into a trailer sulky, or a castered sulky. The switchover is simple, and without tools. In my experience, the castered mode does not work with my 36" Exmark -- sulky drives the mower. The mower is too light, and the footprint is too narrow. By fixing the spindles on the sulky to a stationary position, and pulling one pin, it turns into a trailer. It works great as a trailer. But, like the one is the pictures, it will not permit a full Z-turn. Reversing can only be done very carefully.