creating a stream that passes data through a child process with streams2

suppose I have a function BlackBox. the api is something like this (where | are, in fact, pipes):

inputStream | BlackBox | outputStream

However, BlackBox is actually a wrapper for a require('child_process').spawn, so really it looks like this:

inputStream | BlackBox.Writable -> proc.stdin -> proc.stdout -> BlackBox.Readable | outputStream

I can easily do this with streams1, but I want to understand streams2 and how it is better. Thus, I have the following code so far:

var Duplex = require('stream').Duplex
var spawn = require('child_process').spawn
var util = require('util')

util.inherits(BlackBox, Duplex)

function BlackBox () {
  Duplex.call(this)

  // Example process
  this.proc = spawn('convert', ['-', ':-'])

  var that = this
  this.proc.stdout.on('end', function () {
    that.push(null)
  })
}

BlackBox.prototype._write = function (chunk, encoding, callback) {
  return this.proc.stdin.write(chunk, encoding, callback)
}

BlackBox.prototype.end = function (chunk, encoding, callback) {
  return this.proc.stdin.end(chunk, encoding, callback)
}

BlackBox.prototype._read = function (size) {
  var that = this

  this.proc.stdout.on('readable', function () {
    var chunk = this.read(size)
    if (chunk === null)
      that.push('')
    else
      that.push(chunk)
  })
}

Am I doing anything wrong here?

My main concern is the following excerpt from the documentation on readable._read(size):

When data is available, put it into the read queue by calling readable.push(chunk). If push returns false, then you should stop reading. When _read is called again, you should start pushing more data.

how do i "stop reading"?

To be clear, I want back pressure and throttling to be handled.