They were practicing various technique, principle and footwork in a control manner and speed. Imagine, if the attacker speed up 4 or 5 times, then accident can happen! Since there was NO agreement between the two students, the attacker had to select his way to get through the defender’s defence and the defender had to yield (to give way) to minimize the possibility of getting injured. Obviously, both senior students knows how to hold back the strength so not to hurt each other. After all, they were helping each other in terms of improving technique, footwork, position and get to grip with the hidden Wing Chun principle in a dynamic set up.
Interestingly, if both of them had an agreement as to which direction of the incoming threat is coming from and how the defender absolve and cancel the threat in a certain way, then this agreement carrys on with the next movement and the next and the next….. If this particular video is speeded up, then the outcome would look like a cinematic choreograhied fight scene! This is why sometimes, an actor/actress, who normally not a martial artist can perform incredible feat(obviously assisted by their stunt team/actual martial art tutor on site). In real life, anything can happen. So, be vigilent and be smart!