One more thing, if you still find the old editor comfortable like I do, you can still use this feature. All you need to do is just add the code manually. In your “EDIT HTML” section of creating post, add the code <!– more –> where you want to use the Jump Break feature.
The default code will enable you to have the summary of the post with the “Read More >>” title at the break. If you need to change this to something else you can do it by editing the “Read more” text by clicking Layout and then Edit the Blog Posts widget.
For those who use custom templates, you may not be able to enjoy this feature. If you are using classic templates, blogger probably has some solution for you. Follow the instructions at the bottom of this help article of Blogger.
For more and detailed help of using this feature with customized templates, you can read the post of Bloggerbuster “Read More function to Customized Templates”
Oh yes, you Might as well jump! Like official blogger blog says while announcing this feature.