I think I know the answer, but saw a question on reddit about WC and it made me wonder.
If you use WC to create a piston, and all devices used in that piston are local control, like switches, open/close, etc, then the piston is run locally, right?
Quick WC/ST local vs cloud question
jkp
#2
No, WC only runs in the cloud. 
Right now, the only thing that runs locally are smartlighting automations which use only device type handlers which also run locally. No custom code either smart apps or device type handlers) runs locally. Routines do not run locally. Notifications, including push notifications do not run locally. WebCORE does not run locally.
ptdalen
#3
Makes sense. I knew and understood that custom Device types are not local. For this reason, I try to avoid custom DTH unless they offer me something that I really need. I was not really thinking about the apps, but that makes sense.