[racket] could somebody tell me how this code works?

From: 김태윤 (kty1104 at gmail.com)
Date: Tue Oct 5 07:30:34 EDT 2010

I am a scheme beginner
I am learning big-bang function with the book, how to design programs
but I still have no idea how big-bang function exactly work.

#lang racket
(require 2htdp/image)
(require 2htdp/universe)
(define (handle-draw model) (text model 18 "blue"))
(define (handle-key old-model key) key)
(big-bang ""                           ; initial model is empty string
         (check-with string?)         ; model is a string
         (on-draw handle-draw 400 50) ; display text in 150x50 pixel window
         (on-key handle-key))

could somebody please tell me how this code exactly work?
my guessing is this

there is only one checking function.
if it is true, keep big-bang going.
if it is false, it stops.
the first argument of big-bang "" is going to be sent to handle-draw by
if key pressed, that handle-draw's return value is going to be sent to
handle-key as second argument by on-key
big-bang's first argument "" is changed to the handle-key's return value.
and big-bang start again from the beginning with that value as first

I still have no idea about the old-model argument.
is check-with function must be there?

thanks in advanced
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