FYI - Before trying the technique, keep in mind that because the camera is on when you’re doing this, there is a risk that the electric charge going through the sensor would attract dust particles to the sensor. Especially if you’re taking the lens completely off for longer periods and shooting. Don't do it, if that worries you.
According to Luke Roberts who explains the technique in more detail, " [...] Trying to get light leaks increases the likelihood of dust getting in, but just disconnecting the lens slightly moving it is pretty safe."
The tilt-shift effect would be most likely better achieved with a tilt shift lens or even a lensbaby, but using the technique with a normal lens can get you some really fun, great results.
Here are further resources, tutorials and videos about this technique:
• Freelensing - A Visual Guide - Luke Roberts
• Freelensing Photography Technique - Richard Roche/Gentle Pure Space
• Freelensing Flickr Group
Enjoy the extra bokeh and the tilt shift effects! :)