Welcome hijacker!

You would select the post of the person who highjacked the thread.   Then if there where any replies to that persons post you could select those also. Where it gets messy is when someone post a reply to the orginal post within the replies to the highjacker.

You got the hijaker but missed the replies to him.   No biggy. Now just go back and split the two of them off from this post and then merge them with the hijacker split off topic!

I am getting it better this time but I get a little confused after you make the three choices of splitting only the chosen topic, split from where you choose and the rest and then the last one (I forgot) then you are taken to a new area for three more choices I get the first one which is "new topic" I don't understand the other choices and what they do.

Daryl H

It is all a bit confusing.  I find that the last option "Select posts to split." is the easiest for me to follow.

When I want to split a thread I click on one of the Split Post links anywhere in the thread.  I then give the new thread a title and choose the last option of Select posts to split. 

On the next screen you'll see the post from the old thread on the left and and on the column on the right would be empty unless you happened to have checked some of the messages.  You use the little arrows on each message that you want to move over to the right for the new "split" thread.

For me that method seems to work best.