Hi
You can use FM RSPC_API_CHAIN_START to trigger a processchain manually..
If you BW landscape as multiple application servers and load balancing is configured correctly and you select host the system determines on which server it will run the processchain based on free ressources. If you click server it will run on the application server you are currently connected to. If you only have one then it will be the same anyways.
hope it helps
Martin