When a load is applied on the Tesla turbine shaft slows down, i.e., the relative speed of the discs to the (moving) fluid increases as the fluid, at least initially, preserves its angular momentum. When the rotor is not under load the relative speeds between the "steam compressed spirals" (SCS, the steam spirally rotating between the disks) and the disks are minimal. In a normal operational mode, that peripheral pressure, as Tesla noted, plays a role of BEMF ( Back Electromotive Force), limiting the flow of the incoming stream, and in this way, the Tesla turbine can be said to be self-governing. That belt is most dense, and pressurized, in the periphery as its pressure, when the rotor is not under load, will be not much less than the (incoming) steam pressure. The reaction force, to the steam head pressure, actually builds, relatively quickly, as a steam pressure "belt" along the periphery of the turbine. In the Tesla turbine, considering that there are no blades to be impacted, the mechanics of the reaction forces are different. In this dynamic, the first eddies a loss of the useful energy that can be extracted from the system, and second, as they are in opposite directions, they subtract from the energy of the incoming steam flow. These eddies are created in reaction to the steam impacting (although the minimized angle in the optimal turbine speed) the surface of the blades. In their words, in the optimal regime, the orientation of the blades is trying to minimize the angle (blade pitch) with which the steam is hitting their surface area, as to create smooth steam flow, to try to minimize turbulence. In the bladed steam turbine, the blades must be carefully oriented, in the optimal speed regime of the turbine's work, in such a way as to minimize the angle of the steam attack to the blade surface area.
In standard steam turbines, the steam has to press on the blades for the rotor to extract energy from the speed of the steam, due to the difference between the relative speed of the steam and the blades.
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