From: http://www.chevyhiperformance.com/techarticles/46326/

Another area worth investigating is back-cutting the valves. Flow bench testing has proved that placing a 30-degree back cut on both intake and exhaust valves improves the low- and mid-lift flow of both the intake and exhaust ports. This back cut is placed just inside the 45-degree face of the valve and acts much like the three-angle radius of a valve seat to improve flow. The 30-degree back cut can be accomplished by any competent machine shop and will improve cylinder head airflow even if pocket porting is not included. We have found that back cutting both the intake and exhaust works, but McKenzie decided to back cut only the exhaust side for this test.