In English we say: to see an opening.

to attack where the enemy is unguarded or.to get a chance to step in 例如：她刚和那家伙分手。现在你正好乘虚而入！It means, she just broke up with that guy, it's a good time for you to try to get her love now. 身体太弱，疾病就会乘虚而入。It means, if you are too weak, it's a good chance for diseases to attack you.

take advantage of a weak point it's a neutral words.(predicate)