Board Thread:Questions and Answers/@comment-28154750-20160423014829/@comment-11372432-20160423032750

When a monster is called on another monster via an effect (costs count as effects), then the monster that it was called on becomes part of the soul. You will see this most often with Jackknife decks. Original Jackknife can call another form from deck on top and it'll go into the soul and you can even call another form on top if you have said form in hand to make both part of the soul.

You'll see similar with Jack in Star Dragon World too, but in there, he has effects that can grab cards from the top few cards in deck or from drop into his soul and Arms can move from the field into his soul. And this is on top of having forms that allow one to call on top and put all that in the soul (such as with his impact forms).

If you normal call on top of a monster, the monster beneath will be sent to the drop (not destroyed, just sent). Hope this helps!