The magnets aren't super strong--only something very small like a staple will stick to his fingers, and they don't wipe his hard drive when he uses his laptop or demagnetize his credit cards when he handles them. "The sensation I get near a magnetic field changes from field to field," he says in the IAmA. "AC fields cause the magnets to shake in my fingertips. This causes a similar sensation to bumping your elbow and your fingers going numb. Though, this changes in fields of different frequency or intensity. DC and permanent magnet fields just feel like it's tugging on my finger."