For a video to be engaging and grab the viewers attention it must be emotive at the very start. If your video immediately causes your viewers to 'feel' something they will keep watching.
Once you have their attention and they are emotionally 'charged' you can then deliver the facts you want them to remember.
Once you have done this it's time to ask them to act. This could be to visit your website or simply remember your brand.
And it's really that simple!