How to Pinch Off Plants

Why pinch off plants?

Whenever I've grown selaginella in a terrarium environment it gets very leggy.  The humid conditions seem to kick it into overtime growth and it will takeover a whole terrarium if given the chance.  The remedy appears to be "pinching off" the top growth of each little stem in order to stymy the growth.  This is time-consuming but seems to work?  I wonder how frequently it needs to be pinched off.