I tend to get that slap mostly when stropping a wider blade. If I've used a 5/8...6/8 for a while I get used to a flip that's easy and smooth without much slap on them, and once I go to a 7/8...8/8, etc. and try to flip them at the same speed and rhythm as the narrower blades I usually get the slap. I think it's due to the increased width and flip speed of the edge.
I usually slow down the flip and finish it even more after the start of the stroke reversal.
If it hurts the edge or not....I'm not sure. I remember reading something on the forum about it possibly damaging the edge, but that could have been the flipping on a hone.
Either way, I try to avoid a hard...slapping flip, but that's just me. And, as the "Tip of the Month" says "YMMV" http://straightrazorpalace.com/begin...2015-ymmv.html
Regards,
Howard :)