Talk:Skill leveling Tips/@comment-124.197.115.105-20140504023602/@comment-1328622-20140504070100

It means if you feed something with the exact same skill (name and effect), you may get an extra skill levelup, if the one fused (aka, the one being lost) was at max level, the extra skillup is guaranteed.

i.e. you feed a Wind Weasel to a Dark Elf, since they both have the skill Phantom Sigh, which slightly increases phantom attack, Dark Elf may get an extra skill levelup, so that when she reachs max level, she may have level 12 phantom sigh instead of 11, you can keep doing this, of course, until she reaches the maximum possible skill level of 20, 9 past the normal threshold.

IF the Wind weasel was at max level (level 30), dark elf will get one such levelup, so you only need 9 max level wind weasels to ensure a level 20 phantom sigh on dark elf.

however, given that wind weasel is everywhere, there's not much point in doing that vs just trying your luck. You can usually get a dark elf to level 20 phantom sigh with two or three dozen wind weasels