This goes something like this:

• Look at an interface, it could be a screen you are designing.
• Look away, then look again.

Which parts of the UI feel like they are pushing into your brain - meaning you don't have to think much, you almost instantaneously "get it"?

Example

When I look away and back at this screenshot, The main image stands out, then the other images and the blue highlighted headings stand out.

References

To learn more, see the work of:

Ryan Singer