To produce a Torque figure, the RR needs to have a Resistance factor, decent RR's will have this facility. Strapping a car down to aid the Torque Figure is cheating.
I think they call it tractive effort
rhys straps dont effect hp,,,, aslong as the load you place on the rollers is the same for the power ramp up,, and the ramp down loss measuring
(so no jumping out of the car before it stops if people are sat in the boot
)