Artemis might use a swipe gesture to visually hint at the relation between upvoting and boosting, as shown in the main animation. Here’s a static app mockup with both swipe levels displayed. I’d also imagine that there should be a setting that auto-upvotes posts when boosting.
Yeah, one common implementation is to slightly snap the block to the closest value. It’s not too drastic though, so the switch between icons/backgrounds itself is snappy, but you’re still free to slide the block.
But overall it’s up to developers :)