Momotaro
- Skill Evolution
Simply speaking, the "Skill Evolution" option allows you to "upgrade" the rank of a weapon skill (on a particular weapon), to the next rank, provided that you've maxed it.
** Depending on the rank of the skill, the number of skill levels it takes to max it will differ. If its a Rank 1 skill, it will take 10 levels to max it. If its a Rank 2, then it will take 20 levels to max it. Generally speaking, its "Skill Levels to Max = Rank X 10".
*** The higher Momotaro's "Essence of Evolution" (進化の極意) ability is, the higher the rank that you can evolve the skill to.
**** The higher Momotaro's "Essence of Training" (鍛錬の極意) ability is, the faster you can get those weapon skills to skill up when you use them.
- Skill Extraction
Based on what I could tell from this website, Onigiri Online - Skill Extraction (its in Japanese by the way), you will likely need to have at least more than 10 skill levels before you can attempt to extract any weapon skills.
And upon extracting the weapon skill, you will gain some sort of weapon skill card or something in your inventory. However, your skill level for it will also drop by 9 levels I believe...not to mention that you will lose some in-game coins as payment for the extraction process.
The weapon skill card that you've obtained can then be "slotted" into another weapon of your choice. But must be of the same weapon type, I suspect.
To insert it, just open up your inventory, locate the skill card, select it and hit the "Use" button. The weapon skill will then be inserted into your currently equipped weapon, albeit occupying a random skill slot and forcibly removing its previous owner (the weapon skill that was in that position).
* The higher Momotaro's "Essence of Extraction" (抽出の極意) ability is, the higher the rank of the weapon skill that you can extract.
** Do note that I'm not certain if you can simply extract any weapon skill that you wish to or will they follow a similar criteria like that found in Skill Evolution.