Hi the problem is due to the brain upregulating its serotonin system, while readjusting to having been downregulated by artifical means (The Effexor/antidepressent etc.) during the withdrawal of the drug.
These can feel like 'short burst' electric-shock like sensations which travel or shoot to the extremities, hands and feet, even the heart muscles, they are often accompanied by dizziness. There is information on the internet which documents these withdrawal/discontinuation symptoms.
The brain is readjusting to its own natural production again of serotonin, and getting slightly confused in the process of renatural production of how to do so, so there maybe minor confusion in synapse transmission at the serotonin junction.
The zaps seem to relate to the strength of nerve signals being sent to various parts of the body from the brain, and seeing as serotonin is such a vital chemical, the misfiring in the neurons due to temporary non-availability of it from an outside source (the tablet) anymore,
would seem to indicate why this leads to a temporary heightened overload on the brain, while the brain restacks its serotonin to its natural ryhthmn.
I had the same symptoms during Paxil withdrawal. They were very unpleasant, and most doctors seemed not to understand, nor were able to acknowledge its reason, nor understand really!
It lasted for 2-3 months off, without a taper in my case (apparently helps), and gradually got less, occuring in waves, until it healed.
Grandma.