Scrolling a subwindow in Safari on the iPad

The solution to this wasn't obvious to me, so here's hoping you find this post more quickly than I found out my answer.

If you are surfing with iPad Safari and need to scroll a sub window of some kind (that is, a rectangle within the Web page that itself is scrollable), use two fingers pressed inside the sub window area.

In a regular area of a Web page, using two fingers will scroll the entire page, just like swiping 1 finger, but if your 2 fingers don't move exactly in sync with each other, iPad interprets a 'pinch' or 'expand' and zooms in or out.

However, if you swipe 2 fingers within a scrollable sub window, it scrolls only the contents of the sub window.

Making this a bit more confusing is that you can't really tell when something is a scrollable sub window. The iPad standard seems to be to display scroll bars only when you are actually scrolling.

Which brings me to the User Experience tip of the day for developers: An iPad user may not notice that there is more content underneath the "fold". Users are trained to understand that the existence of a scroll bar means "more" - and also trained to believe that lack of a scroll bar means "there is no more". A hint would go a long way. For a different example, install the USA Today app.