Wikipedia
Flop-transition
In theoretical physics, particularly string theory and M-theory, the notion of a flop-transition is basically the shrinking of a sphere in a Calabi-Yau space to the point of tearing. Based on typical spacetime topology, this is not possible due to mathematical technicalities. On the other hand, Mirror symmetry allows for the mathematical similarity between two distinct Calabi-Yau manifolds. If one undergoes a flop-transition, the mirror of it should result in identical mathematical properties, which it does.