Mochilas shall not be said in a regular fashion or whispered but cried out loud, and preferably accompanied by a specific gesture: dragging one’s hand across the chest diagonally (as shown in the video below). This gesture symbolizes the splitting of something by some sort of knife: the chest represents what's being split and the arm represents the knife.
The popularity of that gesture has made equally relevant the alternative expression ponte la del Puebla (lit. wear the Puebla jersey). This is an allusion to Puebla’s (a city west of Mexico City) soccer team’s uniform, which exhibits a blue stripe diagonally across the chest (shown below).
The following video illustrates both forms: ¡mochilas! and ¡ponte la del Puebla! Notice the intonation. If you want to be discrete or cannot be loud, do the gesture alone. It's equally powerful.
Disambiguation Note: Mochilas is also used to mean mocho or mocha, which stands for someone who is very observant of Catholic principles and therefore not necessarily the most fun to hang out with.