Mind transforms into Vrtti (thought function) and travels through the sense-organ to the place of the Object. The Chaitanya (consiousness) associated with the Vrtti illumines the Object, and the Object gets reflected in the Vritti, i.e. the Vritti takes the form of the Object, by engulfing it. This gives the knowledge of the Objects. This engulfing of the object is called as Phala-vyapti. Or in other words, the Vrtti-vyapti is the one which removes the ignorance and the phala-vyapti is that which illumines the object.
The idea of Rocket to show the vrtti is, when the rocket/bomb is launched it hits the target (removal of ignorance) and blasts, engulfing everything in fire (illumining the object).
When the Vrtti turns inwards towards the Self, since the Self is self-effulgent there is no need for the consiousness associated with the Vrtti to illumine the Self. (Om is just a symbolic representation of the Self, since the Self is not with name and form, it gets reflected as it is). Here there is no phala-vyapti, only vrtti-vyapti takes place, i.e. negation of the ignorance vieling the Self. Since the Self is self-illumined, there is no Phala-vyapti necessary.
Since the Self is self-luminous, like a big fire, the vrtti rocket only removes the ignorance, the vrtti rocket too gets annihilated by that Self. That is why we say the Akhandakara vrtti is ‘sva para nivartya’ – annihilates itself and other vrtti. This is the reason the Akhandakara vrtti, can and will take place only once, and there cannot be a repetetion of Akhandakara vrtti, like mantra japa.