In simple terms. You use a type of ground or air EM...

In simple terms. You use a type of ground or air EM (Electromagnetic device) over your tenement, this sends timed fields of EM into the subsurface/underground. (ie like a magnetic field)

If there is a conductor (an object that has electromagnetic properties) this induces the conductor with the EM waves. The conductor say absorbs these EM waves and creates its own eddy currents in the conductor which exhibits its own electromagnetic field.

The EM survey equipment will pick up this secondary field generated by the conductor. The longer the conductors field takes to be picked up termed (Late Time) generally implies a greater conductance/magnetic property of the conductor. Which exhibits a stronger signal on the EM survey.

The reverse of this is if there is nothing in the ground the initial EM signal field wont be delayed.

Ni-Cu PGE geologies tend to have magnetic properties.