The Pentel EnerGel 2S multi-pen packs two 0.5mm ink refills (in black and red) along with a 0.5mm pencil component. There's nothing fancy as far as the mechanics go for this pen — it's standard fare for multi-pens, which is to be expected for the sub-$10 price. Press down one of the clickers to extend a refill, and press down a different clicker to release and retract the extended refill. In the case of the pencil, you can also press the top of the pen to advance the lead.
Speaking of the pencil component, I've had a bit of trouble when using it. When the pencil is extended, the clicker mechanism is incredibly sensitive to touch, making it retract unexpectedly. It's been fairly common for me to accidentally retract the pencil while I re-grip the pen after depressing the pencil component for use. This can be frustrating, but it seems to work well enough as long as you don't bump the clip while using the pen. Since the clip is the pencil clicker, you can see how this might be problematic.
There's no eraser on this pen to accompany the pencil component, but that's normal for multi-pens of this price range.