If you're familiar with reverse macro, I guess you already have the basic of 'freelensing'. This works the same way and you need to take your lens off the body, hand-held it to add dramatic effects to your snap. You can do tilt-shift, light leaks, macros and a lot of customized blurry effects. This requires a lot of passion, practice and in some cases specific gears.
But you also need to understand, by taking your lens off, you're exposing your sensor and lens mount to dust, mist and other potential risks. So, there's nothing better than a tilt-shift lens if you want to do tilt-shift or a macro lens for macro purpose. This technique is just a cheaper solution to some of them.