Ultrasoft Narrow-Line Seyfert 1 galaxies: What physical parameter ultimately drives the structure and kinematics of their Broad Line Regions?

Recent X-ray observations have advanced the study of ultrasoft Narrow-Line Seyfert 1 galaxies and have revealed strikingly clear correlations between optical emission line properties and the shape of the X-ray continuum. In particular, the strength of the soft X-ray excess relative to the hard X-ray power law appears to be directly related to the `primary eigenvector' of Boroson & Green. Ultrasoft Narrow-Line Seyfert 1s lie toward one extreme of the primary eigenvector, and their extreme X-ray spectral, X-ray variability and optical emission line properties suggest that they have extremal values of a primary physical parameter. We discuss efforts to identify this parameter and the observational evidence that it is the fraction of the Eddington rate at which the supermassive black hole is accreting.