Follistatin is a negative regulatory protein. It works to regulate the body by suppressing the actions of other proteins and hormones.
When overexpressed, it can cause excessive suppression of these proteins. One of these proteins, Myostatin, is itself a negative regulatory of muscle protein synthesis. Thus, increases in follistatin expression indirectly promote muscle protein synthesis by eliminating a 'lock' on protein synthesis.
Follistatin overexposure (typically though injections) mimics myostatin deficiency, which is colloquially known as 'double muscling' and is the cause of hypermuscularity seen in Belgian Blue Cattle.
Follistatin also has interactions with reproductive health, and may have far-reaching effects due to its suppression of proteins that are not myostatin; namely Activins.