How does John Chang use "yin qi" to stop and catch a bullet?

If the right hand is yin, wouldn't that make metal go towards it faster since yin is about absorbing something and yang is about emitting (so repulsing)

it's a good question. Have you ever heard the raise, "Likes to get a rise out of someone?" So certain people like to get other people anger or a "rise out of them" - so that the person can the "suck up" the anger of that other person. So the anger is yang qi but they are using their "yin qi" to suck it up. That is a lower level of what you are talking about. Now on a higher level, the yin qi is literally a black hole. The irony of a black hole is that actually reality, as now defined by Western science, is based in quantum physics due to "time-frequency uncertainty" aka wave-particle duality. So what this means is that as a particle speeds up then its external frequency is actually going up as energy defined by the speed of light. So normally in classical physics we think of momentum as speed plus mass but in the quantum realm it was proven the momentum is inverse to the mass as wavelength (defining time as spatial distance externally). As John Chang states, a person can store up yin qi only as much as they have yang qi built up internally based on celibacy. This is why real qigong masters are so rare since celibacy requires controlling the dream state. So then waking reality becomes another dream state while the "real" truth is beyond the dream state as a literal black hole spacetime vortex. So then based on quantum physics again - the internal frequency goes up as a particle goes into a black hole but the external time SLOWS DOWN. So externally a person or in this case, a "bullet" - never actually makes contact with the black hole or "yin qi" as gravity. So this is called relativistic quantum physics because you have two times - one from the future which is superluminal as the 5th dimension. This is then what "guides" the bullet - and so that time gets shorter and shorter as the internal frequency of the bullet gets bigger and bigger. Then the relativistic external time gets bigger and bigger - meaning slows down as a red shift (which is another way of saying Yin Qi). Then the frequency of the bullet (again based on light as relativistic invariance speed) goes down - as the red shift.

So that is what the qigong master experiences since John Chang is literally "embodying the Emptiness" - through his lower tan t'ien and central channel. For us "normal" people we see the bullet speed up - but John chang experiences the opposite - he sees external time slows down.