What this clause does
The annual gift tax exclusion only applies to gifts of a present interest. A contribution to an irrevocable trust is, by default, a future interest because the beneficiary cannot reach the money right away. The Crummey power solves that by giving each beneficiary a temporary right to withdraw their share of the contribution. The right typically expires after 30 days. After expiration, the contribution stays in the trust and is administered under the trust's normal terms.