Yeah you're pretty much right on there rpm. A pure bluff is when we don't expect to ever win the pot when we're called, but a semi-bluff allows us to win the pot sometimes even if we're called, which is pretty cool.
On really dry flops, semi-bluffing will often be something like you have a lot of runner draws or cards that you can make a +EV turn barrel, or you can even turn something like bottom pair into a bluff if you rarely think it will be good at showdown.