An engine flush is a chemical additive product that contains potent detergents that are designed to clean build up deposits, sludge and other sticky substances from the engine.
The difference between an oil and a flush cleaner is that the oil contains a limited number of detergents (since it is designed to perform various functions) whereas the flush product is designed exclusively for cleaning.
The way to use an engine flush is to pour it into the oil fill port. By mixing with the oil and circulating in the engine, it helps dilute sludge and deposits. Leave the engine run for 10 to 15 minutes and then drain it and add new oil to the port.
By removing the oil, you get rid of the gunk accumulated in your engine as well.
However, keep in mind that old engines that have seals may need that sludge to prevent the oil from escaping through them, working as a barrier against oil leakage, so using a flush to remove that gunk may result in your car leaking oil.
In this case, it is not safe to use a flush cleaner for your engine.