Hi Shaolin-Do
The Filing punch is “somewhat” of an uppercut. One could say this is the Mantis version of an uppercut. I think it might actually qualify for a shovel punch which in my mind has a slighter angle bend at the elbow. The use of uppercut in boxing usually goes to the body either to bring the opponent’s defense low and then attack high. This is not the case in Mantis. Using a Boxing uppercut to attack the opponent’s nose might get yourself into a key lock (figure 4) easily if you are not careful. Remeber Kung Fu unlike boxing, we don’t use glove and are allowed to grab.
The filing punch is like a straight armed uppercut or shovelling punch that is “accompanied” by a lu Shou (grabbing hand). It is fast like a jab and targets the Ren Zhong point that is right underneath the nose. It is by no means a backfist. If that happens, I am affraid that’s just being clumsy in execution. In Mantis training, the purpose of Cho Tsui (filing punch) is to learn to attack with accuracy to one of the 8 strike areas namely the Ren Zhong. This is important as a mantis transmission or lesson if you will. This is the difference between learning from a traditional system and a new found McWhatever (no disrespect intended). Instead of going for a basketful of targets in the proximatety of the intended target randomly, Mantis is highly developed to the point that specific terminologies on techniques are established to convey the messages. It is about the art not the practitioner’s ego [re: instant gratification in hitting something]. It is kind of like playing pool. You call the shots, you’ll have to make the shots.
BTW, it is said that the Japanese soldiers used to test a “corpse” by poking a bayonet into the RenZhong point. A fake vitim could never pass this test and would become a real corpse.
Hope this help
Also thank you for participating in the forum in helping to bring this wonderful art into the lime light. 
Hi Tainan, Brendan and All,
I think the BiMen Jiao, Yuan Yang Jiao, Ao Yu Jiao are different in the way the preceeding moves are.
Bi Men Jiao (close door kick) - passive reactive
If my doors (hands) are closed (obstructed), I do a Bi Men Jiao to get away or turn the table arround.
Yuan Yang Jiao (Mandarin Duck kick) - proactive reactive
I crash against a door (beng) it is closed, so I do a Yuan Yang Jiao to test/take the lower gate or to stop opponent to follow in while I regroup (retreat and return) to attack from another angle.
Ao Yu Jiao (turtoise flipper kick) - Instinctive reactive
This, btw, is found in Mimen Lanjie book as one of the 10 no blocking countering moves “Bu Jiao Bu Jia Shr Lu Po Fa”. Simply put, dodge and kick to the lower gate.
I think Brendan made a very good point in the level of control the kick has. I remember GM Chiu used to demonstrated this on a Iron palm bag bench that is about waist level. He kicked the ledge of the bench with the ball of his foot (he wore very nice Italian leather shoes btw. lol…) with great speed and accuracy. I believe he was 89 at the time. It was amazing to see the level of control that he had.
Warm regards
Mantis108