CAPs are Custom Assembly Patterns or, according to Nick Woolsey, Capped Antispin Patterns.
A very detailed overview of CAPs history and varieties by Drex
Very detailed tutorials on how to do CAPs (C-CAPs to be exact) by Nick
And a tutorial by Drex:
A beautiful pattern with CAPs by Nick
8-step CAPs tutorials:
A nerdy concept that is also used for other double props (double staffs, fans, mini hoops)