For the best description I’ve found: http://news.honda.com/newsandviews/arti ... d=10081-en
I’m not an engineer either, but this is my take so far: The Clarity can run as a serial hybrid (all EV until runs out then gas) or a parallel hybrid (battery supplements gas), the later is like the Accord and other non-plug ins. You choose: EV is the default; toggle the HV button to put you in parallel. Darthpuppy’s succinct response is solid advice on when to use HV vs EV.
ECO and Sport are available in either mode. ECO increases economy at the expense of performance; Sport the opposite. (I haven’t felt the need to use either yet.)
The link above, under POWERTRAIN, describes a passing mode that draws on both gas and electric, engaged by depressing the accelerator pass some click point. This suggests another good reason to preserve some battery by running HV on long trips.
That said, we have been finding FREE charging points using the Honda dash app or Clickpoint phone app. Picking lunch stop near a free recharge station, we got over 100mpg on a recent trip. Enjoy!