Dave explained how the wheelslip works in one of the videos. Both the engine and the tender have motors with encoders. Normally they just run in sync the exact same RPM. In wheelslip, the rear motor in the tender ramps up and moves more encoder pulses over a short but controlled acceleration and then resumes the exact same speed. The rear drives wheels do not have traction tires or grooves for traction tires.
One interesting note that just stuck out to me- both boards are tethered together, but also, both boards have the TMCC/Legacy radio antenna connected and used. Only the tender board has the switch for program/run. Obviously this engine has unique firmware and very specific programming for the boards, but also, each is kind of a complete "engine" with RF sections.