This is a super simple tip that doesn’t really have much to it. A busy indicator is just a visual cue to show the user that something is happening. To use a progress bar in Qt as one, you just need to set the minimum and maximum values to 0. If you do this, you get an infinite progress indicator. This is a better alternative to a progress bar when you have a process in which you cannot calculate the percentage completed, or one that takes a really long, undetermined, amount of time. An example with the Fusion style (that I use for basically any QWidgets these days) yields an animated diagonal stitch pattern:
On Windows 7, you get a nice animated effect as well, but with a smaller colored area moving from left to right infinitely.
I hope the tip helped you out! Happy coding!