Curious where the signal to detonate the pyro bolts and release the hold-down clamps comes from... The rocket about to lift off? Or ground control?
For Apollo, the signal came from a small computer room built inside the mobile launch platform.
Giant holddown arms, whose name exactly describes their function, are positioned on the launcher surface to support and restrain the Saturn V. These arms hold the rocket during the first 8.9 seconds of ignition of its mighty engines while the computer beneath, communicating directly with the computer in the Launch Control Center, verifies the performance of each of the 1,500,000 pounds thrust power plants of the first stage. When all five engines reach full thrust, and only then, the computers release the holddown arms which retract and allow the rocket to rise.
The Kennedy Space Center Story, p. 28
For shuttle it was commanded by the vehicle computers.
At T-31 seconds control of most remaining countdown events was handed over to the vehicle, including SRB ignition and blowing the hold-down post nuts.
See What holds the Space Shuttle orbiter itself stable on the launch pad? for details of the mechanism itself