What "smoking gun" was found in the unexpurgated White House tapes that Nixon was finally forced to release in August 1974, which made it virtually certain that the Senate would impeach him?

The "smoking gun" tape which was released found Nixon guilty of criminal conspiracy and obstruction of justice when he agreed that his administration officials should approach the Director of CIA, its deputy director and ask them to request the Watergate scandal case investigated by the FBI to be halted citing national security as a yardstick for the request.