Follistatin
Follistatin FST — the molecular target that 1 compounds in the wiki act on.
In one line: A natural brake-release for muscle growth — it neutralises the "stop growing" signal myostatin.
Follistatin binds and neutralises myostatin, the protein your body uses to cap how much muscle you build. Less myostatin means the cap comes off and muscles can grow larger — the same reason certain super-muscular cattle and dogs exist. Experimental "myostatin inhibitor" compounds aim to copy this.
Exciting in theory, but human safety is unknown and early trials of related drugs were stopped for side effects. Promising mechanism, unproven and risky in people.