How does passing a function as argument to bind works?

Question

Reading You don't know JS series and got confused with the following snippet

function spread(fn) {
    return Function.apply.bind( fn, null ); // How does this work???
}

Promise.all(
    foo( 10, 20 ) // Returns an array of promises
)
.then(
    spread( function(x,y){
        console.log( x, y );    // 200 599
    } )
)

Trying to understand how passing a function to bind will change its arguments to an array? Note, we are binding a function to apply here and as a result we get substituted apply function for a function with arguments...