This is actually a limitation of the Kayou mechanism.
Because it uses a one-way automatic lead advance system, it can only push the lead forward. When the lead is used up, you have to use the original ballpoint-style twist mechanism to retract and protect the tip.
For a more detailed look at the mechanism, you can refer to this post:
The Kayou mechanism is a very interesting design. Its G2 format makes it especially unique, because it opens up the possibility of converting many existing ballpoint pen bodies into mechanical pencils.
The only downside is that the Kayou mechanism is slightly thicker in diameter than a standard G2 ballpoint refill.（＋0.52mm）
That means we still can’t completely escape the sanding process when adapting it into different pen bodies. The G2 format gives us a lot of possibilities, but some careful fitting is still part of the fun.
Refilling the lead means you need to open up the mod and load a new lead again. (Yes, it only holds one lead at a time… but come on, look at how sexy this thing is! )