A recent observation of the Orion Nebula Cluster with the Chandra X-ray Observatory detected 1075 sources, providing a uniquely large and well-defined sample to study the dependence of magnetic activity on bulk properties for pre-main sequence (PMS) stars descending the Hayashi tracks. The principal result is that X-ray emission does not show the activity-rotation relation which is the dominant correlate of magnetic activity indicators in main sequence stars. Instead, other effects are seen in PMS stars: X-ray luminosities are correlated with stellar bolometric luminosity, mass and size for M < 2-3 M⊙, but the emission drops precipitously in some or all PMS stars with M > 2-3 M⊙. The absence of an X-ray/rotation correlation in PMS stars, and particularly the high X-ray values seen in some very slowly rotating stars, is a clear indication that the mechanisms of magnetic field generation differ from those operating in main sequence stars. The most promising possibility is a turbulent dynamo distributed throughout the convective zone, but other models such as a "supersaturated" α - Ω dynamo or relic core fields are not immediately excluded. The evidence does not support X-ray production in large-scale star-disk magnetic fields. This work is described in detail by Feigelson et al. (2003).