I've often heard, and seen online, that Ip Man had a habit of saying the third empty hand form of Wing Chun "doesn't go out the door". There seem to be many interpretations of this, and a lot of people have wondered what it was that he meant, exactly.
The class involved me coming face to face with my weakness, which is never easy to do. I had spent too much time in a right stance, with my balance over my left leg; the resulting pain was a rather unwelcome reminder of my illness, and it was hard to remain standing. Thankfully, there was an uneven number of students in the class, so I got to take a break as the other members in the trio of which I formed a part took turns alternating between holding the pads and hitting them.
The illness affects one side of the body, but it affects both the arm and leg on that side. My left punch, if not practised diligently, feels pathetic, and so it proved during the lesson. My leg was about to give way under me, and my left punch was weak and inaccurate (I almost clipped my training partner's face a few times, instead of hitting the pad). So, at least part of tonight's lesson was that I need to work on strengthening my left side further: I have made steady progress since the disorder was diagnosed by MRI scan, but I must continue to work on the affected side and not allow the deterioration to escalate.
In the latter part of the class, some of the secrets of third form were revealed, particularly the purpose of the double lap sau within the form. When I asked about how the technique was applicable to knife defence, it prompted the instructor to show variations of follow ups to the initial double lap sau. It was at this point that he repeated Ip Man's famous quote.
As we are soon to be officially recognised as an organisation within the Ving Tsun Athletic Association, Hong Kong, under the leadership of Ip Ching (the younger son of Ip Man), I would trust my instructor's interpretation over any I have seen online: it is likely to have been passed from Ip Man to Ip Ching, who was certainly within the top tier of Ip Man's students.
Third form doesn't go out the door, because it should not be used outside of the classroom. It is the most misunderstood form, to the point that it is where you can spot the authenticity of a particular branch of Wing Chun through their understanding of third form - this is because Ip Man took teaching it as a serious responsibility. The techniques within third form, especially when combined with the power development obtained through practice at that level, can kill. The techniques in first and second form can also kill, but more likely in the "keep hitting until they don't get up again" kind of way. What we are talking about in third form are strikes that can kill in a second.
Ip Man was aware that many of his students were hot headed kids, and that a lot of them liked to go out onto the streets of Hong Kong to test their Wing Chun against other styles of Kung Fu, like Choy Li Fut, for example. There is a story that some of his most promising students threatened to leave his class if he did not teach third form and, Ip Man having the character he did, told them that they knew where the door was, or words to that effect.
Unfortunately, Master Ip never appointed a successor. There is some debate over whether he actually accepted the title of Grandmaster himself, so maybe he thought the title of Grandmaster was not his to pass on. Unfortunately, we now have many individuals claiming they are the rightful Grandmaster of Ip family Wing Chun, so they will necessarily claim that they have learned the full system from Master Ip. I'm not going to question the validity of specific lineages, but we should question the level of maturity which Ip Man judged those students to have.
Another rumour going around is that Ip Man taught Bruce Lee (by far the most famous of his students) the third form in the last months of his life. When Bruce left Hong Kong for America, Master Ip regarded him as a trouble causer, an impudent and quick tempered young man who was too eager to prove himself through fighting. Would he have given Bruce, who he had not seen for some time, a greater ability to use lethal force against an opponent? I will leave the reader to ponder that one.
It's not satisfying, maybe, but the explanation is simply that third form represents a step up in the possible consequences of using the attacks. Ip Man had to tell those few students who had knowledge of the form that it was not to be used outside of the class, however much it had been trained within the class. In reality, faced with an armed opponent, a student of Wing Chun must use whatever allows him or her to survive the encounter, and the use of techniques introduced in the third empty hand form may well be justified. Ip Man's point may have been that these techniques should be reserved for situations where a student's life is endangered, and not used in challenge matches or scuffles.
You may learn the techniques, but hopefully you will never need to use them. Third form doesn't go out the door.