By pressing a Punch or Kick button at the exact moment a hit from the first special move is made, the second special move will automatically begin. When this occurs, a green or pink flash will come from your character at the beginning of the second special.
Example: Ryu, while standing right next to his opponent, throws a Hadouken. The instant the fireball connects, press any kick button, and Ryu will launch into a Tatsumaki Senpuu Kyaku.
Additionally, a Surprise Blow can be used in place of the Special Move for the Momentary Combo by pressing MP + MK when the first Special Move hits.