Node.js callbacks with `when`

callbacks.js

var when = require('when');
var when_node = require('when/node');

// a function that sends `data`
// waits `cycles` milliseconds to simulate network delay
// calls the `callback` after `data` is sent
function send_with_callback(data, cycles, callback) {
  console.log(new Date().toISOString(), 'sending data', data);
  setTimeout(callback, cycles);
}

// create a wrapper function to make
// the send function a promise
var send = when_node.lift(send_with_callback);

// basically you can just chain `then`
// and the callbacks will be sequential
send('foo', 1000)
  .then(function() {
    return send('bar', 500);
  })
  .then(function() {
    return send('baz', 2000);
  })
  .catch(function(e) {
    console.error(e);
  });

Does this wait the amount of time specified for each time you send? or does it send immediately when it's finished and the callback has been called?