With the assistance of a safety pilot, the pilot was flying practice instrument approaches in simulated instrument meteorological conditions. During the second approach, the safety pilot reduced the right engine power to simulate single engine conditions. After raising the hood, the pilot entered the landing flare and reduced the left engine power to idle, at which time the airplane yawed to the left 30 degrees and touched down in the grass. The pilot attempted to straighten the airplane but was unable to do so before impacting trees. The leading edge of both wings sustained substantial damage. The Federal Aviation Administration examined the wreckage and found no mechanical anomalies or failures that would have occurred prior to the accident.