Enemy unit (not Hero or structure) on which it would be casted is removed from this realm and stored somewhere else like parallel universe or something. When ten units are stored they can be released back to this world but now as allies of the caster. Now, some variations are possible, like you can release them at any time or only when certain amount is reached or perhaps there is no limit of how many units can be stored. If it's not hard can someone make it in GUI and place it in this thread for download. I might even try making it myself.