Here we can see the workings of the pen, this is as far as you will get taking apart the pen without destroying it, the spring cased section is the pencil workings and the other part is the pen refill. As you can see the pencil workings enable a "push to advance" method of lead advancement, the pencil section is replenished by filling with new leads in the usual manner with propelling pencils, pop the eraser off the top of the workings and tip them in (see below). The pen is replenished by simply gently pulling the pen refill out of its housing and inserting a new one.
And there it is, the one extra with this pen, an included pen top eraser, usually hidden under a metal ferrule, they appear to be standard size erasers and holders, as witnessed on other mechanical pencils so i guess that it will be easy to replace when it is all used up.And there it is, what you have all been waiting for, the famous pensetc.tk test card. The pen performs perfectly well, all Muji pens seem to work perfectly well from first use and have a very smooth flow, no skipping or troublesome ink flow. As the use of the pen, I think it's an ideal everyday pen, tough due to it's all metal construction, handy with the pen and pencil combination (though if they could make one with two colour pens and a pencil then it would be a must have buy) and cheap enough to buy for everyday use. It feels very substantial in the hand and very comfortable, although as with every all metal pen imagine it can become very slippy for those with hot hands.