So much for the first double arc hand block in Gae-Baek. Now for the second.
Movements 25-28 of Gae-Baek can be used to create a couple wrist and arm locks. The double arc hand "block" can used to set up an S-lock, also called a Z-lock or nikkyo. The way this lock works is similar to the standing armbar (ikkyo). Both require you to pronate the opponent's arm, but whereas in the armbar the opponent's arm is straight, in the S-lock the opponent's elbow is bent. As you bend and force the opponent's wrist towards their body, this forces their elbow upwards. So all you have to do to complete the lock is prevent the opponent's elbow from rising up.
|Sources: coshigould, Black Belt Magazine|
There are other ways to apply the Z-lock. Rather than directly hooking the elbow with your second arc hand, you could trap the opponent's hand as you twist their wrist.
|Another way of applying the S-lock. Source: ExpertVillage|
|Movements 27-28 as an armlock followed by a takedown. Note that in the example the tori controls the right arm, whereas in the form we control the left arm.|
Source: Budo Shingikan School, coushigould
|Source: Your Warrior's Edge|
Because both of these techniques follow from a wrist grab attack, the preceding move in the form -- the twin vertical face punch -- may be used as an initial strike: a quick pop to the face if an opponent grabs your wrist.