A microshock happens when a person and a conducting object acquire different potentials in an electric field. At the instant of coming into contact, the two potentials are equalised by a transfer of charge, and that momentary transfer of charge, concentrated at a single point on the skin, constitutes a microshock.
But what actually happens to the charge - where does it come from and which way does it flow?
Consider the situation where a grounded person touches a floating object (the other situation where a floating person touches a grounded object is exactly analagous). As shown in the left-hand diagram below, before the person touches the object, the electric field induces opposite charges on the two sides of the floating object. There is no net charging, but the charge is separated within the object. For the person, there is a net charging: the field induces a charge on the top of their body, but instead of an equal and opposite charge on the bottom half of the body, charge flows from the ground to cancel this, leaving them with an overall net charge.
The right-hand diagram shows what happens when contact is made. To a first approximation, the charge on the top of the object stays the same. It is the charge on the bottom of the object which is neutralised by charge flowing to and from ground through the person. So the object started out with no net charge and after the contact is made it acquires a net charge.
This is perhaps counter-intuitive. People (including us!) sometimes say the object is "charged" in the electric field and then "discharged" by the person touching it. The correct version is actually the other way round, though we all understand what is meant by this simpler version.
Note that touching the object will in fact change slightly the charges on the top of the object as well because it will alter the shape of the ground plane. And note that because these are alternating field, the sign of all these charges changes every half cycle as the field changes direction.
See more on microshocks in general, quantitative aspects, and what the exposure guidelines say about them.
