Shakti in the context of an aspirant is a complimentary being who is walking the same path with partners supporting each other’s journey towards the higher goal.
It is very rare for the two to join. If they do come together then the sadhana can be greatly accelerated. However, if the shakti is not ready or falls away from the path it can break the sadhana because of the deep inner link between the two.
Normally the coming together of the shakti requires mastery over desires especially sex.