In a rotating machine, the negative sequence current vector rotates in the same direction as the rotor. It is the magnetic flux produced by the negative sequence current that rotates in the reverse direction of the rotor. Thus, the rotor cuts through the flux at twice the synchronous speed, and the induced current in the rotor is twice the line frequency. Regarding measurement of negative sequence, it is measured by the negative sequence filters within the relays.
The net torque is reduced and if full load is still demanded, then the motor will be forced to operate at a higher slip, thus increasing the rotor losses and heat dissipation.