From Online Manual
To split posts from a topic, select the Split button from the top right hand corner of the post where you wish to start the split. You will then be asked to choose one of the following options:
Only split this post: Only the post which you selected will be split and become its own new topic.
Split topic after and including this post: This will split the post which you selected, as well as all the posts which follow it, creating a new topic with those posts.
Select posts to split: This will take you to a screen where you can select individually the posts you want to split out of the topic. You will see two columns. The first one lists all of the posts in the topic which you want to split. To split a post, click on the arrow pointing towards the right column, and you will see this post in the right column where all of the posts to be split are placed. You can undo a post that you had previously selected to be split, by clicking on the arrow pointing to the left column.
Once you have selected the post(s) that you wish to split, you can choose a new title for the topic if you wish, before clicking Split Topic. You then have the option to either go to the Message Index, return to the Origin Topic, or visit the New Topic.