It's not something that has ever happened to me so I can't speak from experience.
It is something I'm wary of, for two reaosns.
One reason is simply because of its association with the hypercharismatic churches that often seem to get heavily into highly emotional and very repetitive songs that can do all sorts of things when a state of euphoria, potentially verging on hypnosis is induced (some have said that what happens in some of these services is effectively a form of hypnosis), and when people start falling like flies I can't help but wonder how much of it is the power of suggestion coupled with a bit of the "me too" desire and maybe a quick shove from the preacher.
The other is that when we see people falling over as if dead in the Bible they seem to fall forwards, on their faces. Ezekiel fell on his face (Eze 1:28), Daniel was on his face (Dan 10:8-9), when the voice from the clouds announced "This is my son, in whom I am pleased" the disciples fell on their faces (Matt 17:6). But when people are "slain in the spirit" they always seem to fall backwards.
As far as a Scriptural precedent goes it's hard to say, other than to observe that people falling over backwards doesn't fit very well with recorded instances of people falling over forwards. I also can't help thinking that even if the claims in favor of the phenomenon are valid and God truly is doing a work in someone's life I wouldn't expect to find large numbers of people in one place dropping like flies, I'd expect it to be a far more selective phenomenon. I certainly wouldn't expect speakers to be making backhanded jokes about it, about people standing behind someone to be in danger etc. To me that sort of silliness suggests the person is treating the Holy Spirit like some kind of personal power trip, and talking as if the Holy Spirit is a power they can wield at their own will.
That said I'd want to know more about it to actively reject it, but from what I can see I'd be cautious of it.