Here is what I think happens - stimulated by the ideas of Martyn Lloyd Jones, but going considerably beyond what he said.
In a (genuine) Christian revival, God opens the minds of people - in a certain place, and for a limited time - to the workings of the Holy Ghost.
In practice, this means that he opens minds to spiritual influences.
But God does not influence free will.
So, in a revival, minds are opened to spiritual influences both divine and demonic and each person must choose.
So a revival is characterized by new insights, new truth; and also by new delusions and deceptions - by great good, and by amplified evil.
And these effects are seen not only in different people (goodies and baddies) but in the same mind - a person may become both more divinely spiritual and more wicked.
The longer a revival continues, the more that the dark forces will tend to prevail - in aggregate and in individuals.
This dual-effect is the reason why revival is not a permanent state of human society, why revival is limited in time and space.
So, God chooses a time and place as ripe for revival (for reasons which may only be later apparent) and opens minds - but then does not control what happens from that point, because what happens is the result of human agency, and numerous individual human choices between the spiritual forces to which minds are opened.
Think of the Eastern USA in the early 19th century when spiritual experiences became commonplace and many Christian denominations were revitalized; and at least five major 'religions' began: the Mormons, Seventh Day Adventists, Jehovah's Witnesses, Christian Scientists and New England Transcendentalists.
Minds were opened, choices made. Good and evil became stronger - both within and between people. Then minds were returned to 'normal' sensitivity.
The consequences unfolded through the following decades. Much good eventuated, much wickedness too.
In sum, modern Western spirituality was established at that time.
The possibilities of revival are constrained by the humans involved.
A revival in unpromising soil is a high risk business - indeed the more necessary a revival is, the less likely it is to succeed - because the more likely it is that people will make the wrong choices, the bad choices.
The mini-revival of the mid-1960s on the West Coast of the USA happened at a time when revival was very much needed, due to the rapid corruption of Western Society - yet that corruption acted to influence people such that the 1960s revival was mostly anti-Christian.
A people already advanced in corruption had their minds opened by God and were were given the chance to choose the influence of the Holy Ghost.
But, on the whole, most of them made the wrong choices, most chose Satan in preference to Christ - so a lot of evil and not much good came from the mid-twentieth century religious revival.
On the one hand, we need periodic revivals - such as the Reformation; on the other hand, the later the days, the less chance that revival will will do more good than harm.
Thus, we have never needed a Christian revival more than we do now; yet the chances that a revival would lead to more good than evil are lower than they ever have been - if modern minds were opened to spiritual influences, they are more likely than ever before to choose the demonic in preference to the divine.
Still, despite the highly unfavourable risk profile, I expect a Christian revival will come because it is our only chance - be prepared...