Griffiths transversality ($\nabla F^p\subseteq F^{p-1}\otimes {\Omega }_S^1$)

In a variation of Hodge structure, differentiation lowers the filtration index by at most one. In the relative de Rham construction, an absolute form of degree at least $p$ contributes, after extracting one base differential, a relative form of degree at least $p-1$. This filtration shift proves the condition for geometric variations.