From Xojo Documentation
Immediately sends the contents of internal write buffers to disk or to the output stream.
This function can be useful in point-to-point communication over sockets and similar connections: To optimize for transmission performance, some types of output streams try to collect small pieces of written data into one larger piece for sending instead of sending each piece out individually. By calling Flush, the data collection is stopped and the data is sent without further delay, reducing latency.
When using this on a stream that ends up as a file on disk, it is useful, too: Any short parts of previously written data are written to disk right away, ensuring the data is actually on disk if the application terminates abruptly, e.g. due to a crash.
Avoid calling this method too often. For example, do not call it between successive Write calls because you’ll slow down performance without getting much benefit.
A typical use case would look like this: