Given the fact all electromagnetic fields are continuous, and can
permeate through any media, the electromagnetic force will act upon
metal piercings even if you are lying on the bed.
The bed has electromagnetic permeability (nu), more than zero.
There are two sources of electromagnetic field: magnets in the bed and magnets in the floor( not on the floor).
When arranged properly, the interference of their magnetic fields can
result in the resultant magnetic field dying down outside the space
determined by the floor and the bed.
I am really interested in how they do this.