Oops, I should have included that straight away.
To split a thread, you click the

Split button.
The thread comes up, with a "box" for a new topic title on top, and with a drop down menu, where you can choose where the new, split off/out topic should go.
On the right of each post you will find "boxes" to click ("tick").
Here you have two options.

If you want to split out one or several posts -- say post 2,4 and 6 -- and make a new topic out of them, you click each post, and then you click the button that says
[Split selected posts].
The selected posts will be made into one topic/thread, and moved to the forum you chose. And the old topic will remain, containing posts 1,3,5 and 7.

If the topic "veered off" at a certain point and quite simply needs to be split in two, from a certain post and down, you can just choose to mark the first post you want to move, and click the button called
[Split from selected post].
That post, and all posts below it, will now be made into a separate topic, and moved to the Forum you chose.
FoxLeonard