To flush the toilet for a second time after laying the load to ensure there is no residual shit left behind.
Rob, taking a shit at his friends house, took an extra Safety Flush to flush the residual shit.
The act of a pre-emptive flush of the toilet BEFORE sitting down. Normally to ensure that the toilet will flush when needed and not back up. Usually performed when expecting a turd on exceptional volume and density, such as a bowl winder.
After that last bowl of chili, I decided to use a safety flush before I dropped a payload.