The certificated flight instructor (CFI) and student pilot were returning to the airport at the conclusion of a lesson. The CFI stated that the student was flying the airplane, and the landing approach was "stable and normal except for the fact that it was low." The CFI took control of the airplane, added power to avoid landing on the runway's displaced threshold, and transferred control back to the student. The CFI stated that the student reduced power "abruptly," and that the airplane's nose "dropped" and impacted the runway before he could recover. The CFI performed an aborted landing, and subsequently landed normally. Later inspection of the airplane revealed damage to the engine firewall. The CFI stated that there were no mechanical malfunctions or anomalies with the airplane.