Grassmann's Law is the tendency for successive aspirates in Proto-Indo-European to dissimilate. This happens in Greek and Indic.
Example:
*bhewdh- "be aware"
> Greek πευθ- (not *φευθ-)
> Sanskrit bodh- (not *bhodh-)
This law also applies to the rough breathing in Greek, thus εχω and not hεχω for PIE *seg'hō.
In Greek, Grassmann's Law remains active for θ (meaning, it will lose aspiration regularly in the presence of another aspirate, and retain it otherwise).
Forms with φ and χ, however, are generally levelled to π and κ, even in forms of a word that no longer have other aspirates in them, e.g. παχυς "thick" has regular π for PIE bhnghu-, but in πασσων "thicker" Grassmann's Law would have left it *φασσων.