Your toilet doesn’t require pressure or running water to function. It works through gravity. You just need access to water to fill the bowl – which of course may be easier said than done if the water is turned off! Any water will do. You may have to ask a neighbor to help you out. If you know in advance you are going to have a plumbing repair, fill a couple of buckets with water in readiness.
Quickly dump about a gallon of water into the bowl. If you pour the water too slowly, nothing will happen. If you don’t put enough water in, nothing will happen. When you get it just right – Bingo! Royal Flush!