This tool is made to help you do better presentations. Here are some tips :
Each time you look at the projector, you show your butt to the audience. The audience is your audience. They came to your presentation to listen to you, you have prepared this presentation for them. Be with them, respect them, look at them, speak to them.
When you look at your mobile device, you are not speaking to your audience. When you look at your mobile, it must be a quick look, to remember the main point of your slide. If you look too much at it, it will look like a conversation between you and your brand new mobile, not between you and your audience.
If you can't read your slide on your device, then chances are that people at the back for the room can't either. Use your mobile like an indicator of the clarity of your slides. If you write too many things, you won't be able to remember the main idea and will be tempted to read your slide, on the projector (see the first two tips). More importantly, your audience will take time to read the slide and won't listen to you during this time.
More tips can be found in the book "Presentation Patterns"