I've intentionally practiced over the years to have a wide variety of tools for such things. For a task like that up to a certain threshold I would just visualize. Like PolarBear says, I wouldn't know what I do if I haven't intentionally studied and worked on slowing it down (toss in something Heisenbergy here) enough to tell it is primarily visual. As complexity increases, I use visual chunking with number patterns, and in the past have done pretty good size lists. Put on some background music and my ability will plummet to almost nothing.
I learned long ago that verbal rehearsal was weak and fragile for me.
I've also used tools like memory castles to visually store strings of information using visual imagery combined with semantic story elements. But lots of those tools are rusty.
This is also a problem with retests on such things. Your gifted kid may worry over the problem long after the testing and have a strategy for the next time. Or they may intiate a strategy mid-task and completely obfuscated the underlying results.
Fun stuff.
