An example of Alternating Focus is to draw a line between 1 and A, 2 and B, etc. etc.
In this final example of scattered numbers and letters (PLEASE SWITCH TO THE GRAPHIC VERSION), the task is to draw a line between number 1 and A; number 2 and B; number 3 and C; etc. etc.
Note that if we connected only the numbers (or the letters) it would have been a sustained focus task.