After you've created that first full cross-over, grab a section of hair right next to the braid and incorporate it into the plait, from the top. After you braid this piece in, leave behind the last section, so it's no longer a part of the braid. Replace that strand with the piece of hair that is between your two remaining strands, adding it into the braid. Grab another adjacent section and weave it into the plait, continuing the pattern of adding in-between sections and leaving behind back pieces. This is what creates the waterfall effect.

Photographed by Guang Xu