The child powershell is a workaround for an unidentified bug which causes colours to get messed up after using git in the same console window. However, loading profiles on the child is a major slowdown when starting the server, and for no good reason.
This allows specifying a PocketMine-MP PHP file to execute (phar or source) to use to run the server, like start.sh. If not specified, auto-detection is used.