[Klashikari]

Originally Posted by Moon Eclipse

It's hard to understand what you're trying to write. Possibly because I'm tired, but do you mean to say:

Spoiler:

That you cannot have someone (whose name you've written in the book) affect someone else's life line (that is to say terminate it) in the instructions you leave in the book leading up to the original person's (the person whose name you've wrintten in the book) death?

roughly : yes.
i'm aware, this kind of thesis was pretty solid, the explanation was, unfortunately, explained much later in the manga. (i don't think it was such huge spoiler but meh...)
IIRC, it was never really explained how Light found about this rule (if any DN manga reader can answer this, please do so.)

the correct rule is :

Spoiler:

HOW TO USE XXVI

Even though only one name is written in the DEATH NOTE，if it influences and causes other humans that are not written in it to die，the victim’s cause of death will be a heart attack.