nicely explain the technical view about the boxes by sr114.
i want to add something more...
now in orginal nirvana, GORDON'S PIVOT METHOD is used. in this method piv box continuosly reforming when a new HH,LL,HL,LH take place.
and in modified nirvana, KWIKPOP'S PIVOT METHOD is used. in this method once a piv box formed, suppose HH (pink BOX) , next it only try to find LL (blue BOX). after few bars confirmation it appear.
now for me as i take as it visual trading, kwikpop's piv i like. also just a info- mrtq13 use kwilpop piv in his trading system.