Thanks to Redditor /u/doboksquawk‘s keen eye we have more information on U-Nam’s movement #23. On page 115 (page 67 in Master Vitale’s PDF) there are the following photos describing the Side Snap Kick:
This seems to be somewhat inline with what Shotokan karate refers to as a Side Snap Kick. The execution presented in the linked video is somewhat in-between how Taekwon-Doists would execute a Front Snap Kick and a Side Piercing Kick, which makes since considering the choice of name. Yet the description in the photos above seem to be more inline with a Side Piercing Kick or Side Thrusting Kick as described by the Encyclopedia’s Volume 4 (Leg Techniques).
Looking to the Encyclopedia for images that approximate those found in the 1959 book turns up the following on pages 18-20 (Side Piercing Kick) and 28-29 (Side Thrusting Kick):
Interestingly, page 20 shows a Bending Stance A with both fists on the supporting leg’s hip as being an incorrect execution of a Side Piercing Kick due to no guard. This is pretty clear evidence that U-Nam movement #23’s technique was discarded due to the lack of a guard. Also telling is the description of the basic principles of a Side Piercing Kick (from page 17):
- The attacking tool must reach the target in a straight line in a revolving motion.
- Regardless of the stance, the foot sword must be brought to the inner knee joint of the stationary leg prior to delivering the kick.
This seems to describe exactly what is going on in the 1959 depiction! Continuing our search through the Encyclopedia, the description for a Side Thrusting Kick also seems to describe the motion of the kicking leg in a similar fashion to the 1959 depiction.
So… what is U-Name Movement #23? From the information presented above, it could well be a kick that is no longer practiced in Taekwon-Do called a Side Snap Kick. But what is more likely is that “Side Snap Kick” was terminology that was later changed to either “Side Piercing Kick” or “Side Thrusting Kick”. The image isn’t clear enough to determine if the attacking tool is the ball of the foot (as in a Side Thrusting Kick) for the foot sword (as in a Side Piercing Kick) but a best guess from what can be seen plus the research detailed above would seem to indicate a Side Piercing Kick.
Hence, U-Nam’s movement #23 should be executed as a Side Piercing Kick with a Side Fist executed over the kicking leg.