To spam taunt you must press the taunt button (usually g or f) very quickly whilst holding down a direction (usually 'w', 'a', 's' or 'd').

I thought you'd ask that!

Spamming taunt will distract your opponent. This will always give you the advantage in a fight.

Also, it is used in virtually every fight at esport level. As a result your opponent will know that you are familiar with the pro fighting scene and will be extremely intimidated.

As the saying goes, “If you want to control someone, all you have to do is to make them feel afraid.”