This is a very simple task. In modern ballooning, it is almost always done virtually because of the amount of resources needed to accurately measure time with physical markers, although theoretically it can be done. For the purposes of this page, I'll be explaining the virtual versions. 

The shortest flight is a time based task. It defines two points, a reference point and the point that will be measured. The pilots can manipulate the point that will be measured, which is typically a virtual logger mark. The goal is to ensure that the distance from the reference to the measured point is as small as possible.

For example, if point A is the reference, and pilots can not press their virtual marker B until they are in a set airspace, they must fly to the point within that airspace that minimizes the distance between A and B.

Think of this one as the virtual version of a GBM.

Because this is another virtual task, enjoy the balloon picture.