The original Kenpo salutation dates back to the boxer rebellion in China. At that time, the salutation was used as a gesture to show that you where one of the individuals fighting to bring back the Ming dynasty. The left hand over right fist represented the sun and the moon, which in Chinese characters formed the symbol of the Ming dynasty.
However, the salutation as executed today by American Kenpo practitioners is longer than the original and now only represents a linkage to its past heritage of Kenpo. The reason the modern-day salutation is longer than the original is that SGM Parker added a "new" series of maneuvers to the end of the original salutation. This change was to represent a merging of the modern martial arts with those of the past.
Beginning with Long Form Three, each form starts their execution from an attention stance. Therefore, the salutation for each of these higher forms is executed completely through to the closing attention stance, at which point the form begins.
One thing to note about the salutation and its execution with forms; the salutation is always appended to both the beginning and the end of the form. But the signing gesture is only added at the beginning of the form.