The null hypothesis is the estimated latent growth model and the h1 unrestricted alternative is the usual unrestricted variance covariance model. The main advantage of the bootstrap method is that it does not depend on the assumptions of normality.
Enders, C. K. (2002). Applying the Bollen–Stine bootstrap for goodness-of-fit measures to structural equation models with missing data. Multivariate Behavioral Research, 37, 359–377.