How does the [].push.apply work?

can someone please explain me how does this line of code work.

[].push.apply(perms, permutation(arr.slice(0), start + 1, last));

This function generates an array of all permutations of an input array;

var permutation = function(arr, start, last){
  var length = arr.length;

  if(!start){
    start = 0;
  }

  if(!last){
    last = length - 1;
  }

  if( last === start){
    return [arr];
  }

  var temp;
  var perms = [];

  for(var i = start; i < length; i++){
    swapIndex(arr, i, start);
    console.log(arr);

    [].push.apply(perms, permutation(arr.slice(0), start + 1, last)); 

    swapIndex(arr, i, start);
  }

  return perms;
};