What exactly does t.annotate do?

When annotation mode is turned on

This

function printThing(thing: string) {
}

becomes

import t from "flow-runtime";
function printThing(thing) {}
t.annotate(printThing, t.function(t.param("thing", t.string())));

However while I totally understand what warn and assert does, I can’t seem to find any docs on just what t.annotate actually do. Can someone help me point me to the right place to look?

[phpnode]

type StringMaker = (id: string) => string;

function makeNumber (id: number): number {
 return id; 
}

const fn: StringMaker = makeNumber; // fails when annotate is on, passes when off.

[tonyxiao]

Super helpful, thanks so much!