Here you go, from the repo:

  const visitAd = function (ad) {
    function timeoutError(xhr) {
      return onVisitError.call(xhr, {
        type: 'timeout'
      });
    }

    const url = ad && ad.targetUrl, now = markActivity();

    // tell menu/vault we have a new attempt
    broadcast({
      what: 'adAttempt',
      ad: ad
    });

    if (xhr) {

      if (xhr.delegate.attemptedTs) {

        const elapsed = (now - xhr.delegate.attemptedTs);

        // TODO: why does this happen... a redirect?
        warn('[TRYING] Attempt to reuse xhr from ' + elapsed + " ms ago");

        if (elapsed > visitTimeout)
          timeoutError();
      }
      else {

        warn('[TRYING] Attempt to reuse xhr with no attemptedTs!!', xhr);
      }
    }

    ad.attempts++;
    ad.attemptedTs = now;

    if (!validateTarget(ad)) return deleteAd(ad);

    return sendXhr(ad);
    // return openAdInNewTab(ad);
    // return popUnderAd(ad)
  };

  const sendXhr = function (ad) {

    // if we've parsed an obfuscated target, use it
    const target = ad.parsedTargetUrl || ad.targetUrl;

    log('[TRYING] ' + adinfo(ad), ad.targetUrl);

    xhr = new XMLHttpRequest();

    try {
      xhr.open('get', target, true);
      xhr.withCredentials = true;
      xhr.delegate = ad;
      xhr.timeout = visitTimeout;
      xhr.onload = onVisitResponse;
      xhr.onerror = onVisitError;
      xhr.ontimeout = onVisitError;
      xhr.responseType = ''; // 'document'?;
      xhr.send();
    } catch (e) {
      onVisitError.call(xhr, e);
    }
  }

  const onVisitResponse = function () {

    this.onload = this.onerror = this.ontimeout = null;

    markActivity();

    const ad = this.delegate;

    if (!ad) {

      return err('Request received without Ad: ' + this.responseURL);
    }

    if (!ad.id) {

      return warn("Visit response from deleted ad! ", ad);
    }

    ad.attemptedTs = 0; // reset as visit no longer in progress

    const status = this.status || 200, html = this.responseText;

    if (failAllVisits || status < 200 || status >= 300) {
      return onVisitError.call(this, {
        status: status,
        responseText: html
      });
    }

    try {

      if (!isFacebookExternal(this, ad)) {

        updateAdOnSuccess(this, ad, parseTitle(this));
      }

    } catch (e) {

      warn(e.message);
    }

    xhr = null; // end the visit
  };

That’s pretty much it! Let me know if it doesn’t make sense, I can annotate it