Great film short, it looked like a lot of fun to make, my compliments to your
Sifu!
Now, as for this "mystery" technique:
Historically,
Ching soldiers were primarily trained in the
Bak Pai Cheung Kiu/ "Northern Long-Bridge System". The
Chin Ji Sau Fat/ "Thousand-Character Technique" was one of the tactical methods specifically designed to counter this.
The idea being, that when the
Ching Enemy would attack with any straight development to the middle/ lower gate, the
Hung Gar Freedom Fighter then slashed it down with the diagonal lead hand
Pek Sau/ "Chop",
Yat Peet/ "First stroke of the Character
Chin" (千).
Now, whether the
Ching Enemy threw again or not, the
Seung Gung Sau Fat/ "Double Bow Hand Technique" was coming from our Freedom Fighter directly,
Yat Waak/ "Second Stroke of the Character
Chin" (千).
This is because, our
Hung Fighter knew that the
Ching Soldiers had been busy training too, to roll off of
Hung deflections with the
Gwa Da Lin Wan Kyun/ "Hang Punch/ Strike Continuous Attack".
Therefore, as the
Ching Enemy attempted to roll over our
Hung Fighter's defense with his
Gwa Chui/ "Hanging Punch", our Freedom Fighter simply followed it upward, trapping it above, as the palm was delivered horizontally to the
Hei Mun/ "Air Gate" beneath,
Kwong Lau Kyut Ba/ "Torrential Flood-Waters Weaken the Dam".
Now, regardless of which hand which gate, the
Ching Enemy was then finished directly by our
Hung Gar Freedom Fighter's
Seung Gwa Chui Fat/ "Double Hang Punch Technique", delivered vertically,
Yat Kei, as the "Third (and final) Stroke of the Character
Chin" (千).
The "Rear Hand Lift, Lead Hand Strike" option shown can be readily seen in both Mok Kweilaan and Tang Fong school presentations. Certainly, Lam Saiwing schools do use this variation, albeit typically maintained in other sets and drills.
That being said, the "Rear Hand Lift, Lead Hand Strike" option is more selective, requiring that one applies this from the outside gate. This is primarily because of the opponent's other hand (as Subitai points out).
Conversely, the "Lead-Hand Lift, Rear Hand Strike" application works equally well from both the inside and the outside gate.
Both are examples of
Fan Lau Sau/ "Leaking Hands", a technique as applicable today as it was 100 - plus years ago.
Best,
Michael