Hello,
According to javadoc:

Allows connections to be flushed upon return to the pool. It's not enabled by default.

According to code and experience:

If flushOnClose is configured for a Pool  the Pool does not grow bigger than minSize?

It  also means, that flushing only occurs if one more than minSize connections are used. 
The Code can be found in ConnectionPool.java:

  int currentSize = allConnections.size();
  if ( currentSize > configuration.maxSize() 
       || configuration.flushOnClose() 
          && currentSize > configuration.minSize() ) {
      ....