What's the `checkScrollViewContentOffset` method do?

[qhhonx]

Firstly, thanks your fantastic works.
I checked codes and can not understand checkScrollViewContentOffset did clearly. When I commented those invocations simply, it seems that no weird actions occur.
Could you explain the related codes?

[itouch2]

If you zoom in the picture and move it to the edge of the bounds, then you change the slide to rotate it. You will find the picture will beyond the bounds. checkScrollViewContentOffset is used to check such case.