A more thorough explanation:

A simultaneous-death clause is a legal provision that is included in a will or trust. It states that if the testator (the person who made the will) and the beneficiary (the person who is supposed to receive the assets) die at the same time or in a way that makes it impossible to determine who died first, the testator is presumed to have survived the beneficiary.

For example, if a husband and wife are in a car accident and both die, the simultaneous-death clause would assume that the husband outlived the wife. This is important because it affects how the assets are distributed. If the wife was supposed to inherit everything, but the simultaneous-death clause assumes that she died first, then her assets would be distributed according to her will or trust.

There is often an exception to this rule if the beneficiary is the testator's spouse. In this case, the spouse with the smaller estate is presumed to have survived. For example, if a husband and wife both die in a plane crash and they have similar-sized estates, the simultaneous-death clause would assume that the wife outlived the husband. However, if the husband had a much smaller estate than the wife, the clause would assume that he outlived her.