There should be no damage to the engine, but the torque produced could cause damage to the tailboom or the drivetrain back toward the tailrotor. As long as the engine doesn't go above idle, the tiedown should stay intact, but if the rope is badly frayed, or the knot is poorly tied, the blades could suddenly come free, & then the likely result is a turnover, from the sudden torque and the imbalance from the weight on the end of one blade.