How does IE6/7 "back button" support work?

newtwitter-hashchange.js

// From http://a5.twimg.com/a/1284588078/javascripts/phoenix.bundle.js

(function (B) {
  var A = (function () {
    var E = {
      select: "input",
      change: "input",
      submit: "form",
      reset: "form",
      error: "img",
      load: "img",
      abort: "img"
    },
        C = {};

    function D(F, I) {
      var H = (arguments.length == 1);
      if (H && C[F]) {
        return C[F]
      }
      I = I || document.createElement(E[F] || "div");
      F = "on" + F;
      var G = (F in I);
      if (!G && I.setAttribute) {
        I.setAttribute(F, "return;");
        G = typeof I[F] == "function"
      }
      I = null;
      return H ? (C[F] = G) : G
    }
    return D
  })();
  B.fn.hashchange = function (D) {
    if (A("hashchange", window) && (document.documentMode === undefined || document.documentMode > 7)) {
      this.bind("hashchange", D)
    } else {
      var C = location.hash;
      setInterval(function () {
        if (C !== location.hash) {
          D(B.Event("hashchange"));
          C = location.hash
        }
      }, 250)
    }
    return this
  }
})(jQuery);