Class stat floors: Raise Defense from 6 to 10 and keep it after switching back
Armored Knight first raises base Defense to 10, then adds its class modifier of 4. Switching back removes the modifier but keeps the improved base.
Original video: Still waiting for stat gains? A Fortune's Weave class-floor trick for easy rare stats! | Bilibili creator -Yuno Anna-
Certification floors improve base stats. In this example, Defense rises from 6 to 10, and those 4 points remain after returning to the original class. Armored Knight's additional class modifier is removed.
Separate the Defense floor from the class bonus
A magic unit with only 6 Defense switches to 
Armored Knight and shows 14 on the stat screen. Break those 14 points down as follows:
Four Defense remains after switching back
Return to the original class, and Armored Knight's 4 Defense disappears, but the base remains 10. Compared with the initial 6, one certification seal earns 4 base Defense.
That is using a class's stat floor: cover a weakness through the target class's minimum, then return to the class you want to play.