Acceleration of estate

What does Acceleration of estate mean in American Law?

The definition of Acceleration of estate in the law of the United States, as defined by the lexicographer Arthur Leff in his legal dictionary is:

When a future estate vests more quickly than stipulated because a prior estate failed to come into existence, or terminated prematurely, it is said to have “accelerated.” For instance, in a testamentary conveyance “to wife for life, then to X,” if the wife renounces the will and elects to take her dower or statutory share instead, X will take immediately as if the wife had died, and X’s estate would be said to have accelerated. But an estate is not “accelerated” just because a prior estate did not last as long as expected, e.g., when a young and healthy life tenant dies shortly after coming into his life estate.