This technique won't send your audience to the snack bar for a soda, but it can be fun.
Some clever effects are simple to create, but not necessarily something you'd think of on your own. Including a subliminal message is one of those tricks. Creating it is simple -- thinking of it is the hard part.
To include a subliminal message, insert the appropriate text or graphic object. Then, right-click the item and choose Custom Animation. Click Animation Effects (the yellow star) and choose Entrance. From the resulting submenu, select Flash Once. That's it! During the presentation, PowerPoint will quickly flash the item, just once, when that slide is current.
Now, the appropriateness of such a technique matters. For the most part, you'll use this trick in a humorous way -- and used correctly, it's effective and hilarious. Just keep in mind that humor is subjective and use good judgment. I don't recommend using this technique during a serious presentation. Also, be sure you know your audience well and that they're open to the message and method.