A couple years ago I was talking to someone about double turnovers. Apparently we had exhausted every potentially interesting subject of conversation. The conversation centered around the question of what type of double lift/turnover looked the most "normal."
For the most part, I think the majority of double turnovers seem fairly above-board as long as they're performed well. However, I sometimes think magicians shoot themselves in the foot when they do anything more than turn the card over. Sometimes they do this weird flexion thing with the double that no one has ever done with a single card.
(That's not me. That from someone's video teaching the DL.)
Or sometimes they'll do a flourishy double where it's held between the corners and rotates around multiple times or swivels out of the deck or something like that. I think they believe they are emphasizing the singularity of the card with these moves, but in my opinion they are just bringing attention to a moment that should have passed by without note. If a spectator is suspicious of how many cards you turned over, then your technique is bad, or you're performing for someone who knows about double lifts, in which case your flourishy doubles aren't fooling anyone.
We often hear when learning sleights that we should do the action as if we were doing it for real. That is to say, when you pretend to take the coin or pretend to pick up the object, it should look like when you really take the coin or really pick up the object. So when you do a double turnover, it should look like when you do a single turnover. But there isn't really such a thing as a "single turnover." I mean it doesn't have an analogue in the real world. Holding a deck and turning the top card face up and placing it back on top of the deck isn't something that's really done outside of magic. (Perhaps there is a card game where this is done, but I can't think of it.)
So that left me with the question: If I wanted to handle cards in the way a non-card-handler would, what should my double turnover look like? Not that I was going to mimic them exactly. I don't want to look clumsy with cards. But just as a starting point, I wondered how a person who hadn't practiced this 10,000 times would naturally turn a card over on the deck.
I decided to find out. And so, in the intervening months and years, whenever I was with someone who wasn't proficient with a deck of cards—and whenever I remembered to do it—I would take a video of them turning the top card face up on top of the deck. I didn't want them to think too much about that action, so I built it into a trick. That way I could take my phone out to record "something I want to try out," and at a certain point during the trick I'd say, "Take the top card, turn it over, and put it back on top." And thus I had a bunch of "real" people doing the action with a single card that we pretend to do with a double.
The video below collects those clips.
So what did I take away from this? Well, the main thing I noticed is that only two of those people do anything close to what I do with my standard double turnover. That is to say, only two people took the card off the side of the deck and turned it over sideways. (I should note that those were two of the only people in this video who "occasionally" play cards.)
It seems the most "natural" way to do a turnover for non-card-handlers is to slide the card from the back (or front) and turn it over end-for-end. When I noticed this trend as I began to collect the clips I thought, "Well, I'm not going to start doing a double turnover like that, of course." But then I started playing around with it, and it began to feel very natural and innocent to me (when I could make myself forget everything I knew about what a double turnover is "supposed" to look like). So now that is the double I use when I'm performing for non-card-handlers (and it actually flew by a number of magicians I tried it on as well—presumably because they were thinking, "No one would do a double like that.")
(When I perform for people who do play cards regularly, I do a more traditional push off double, because I have a feeling that would look the most normal to them.)
Is this meaningless fine-tuning of a sleight? Possibly. I don't think it's something people notice. I don't think they say, "That looks like a very fair, natural way to turn over the top card." They're non-card-handlers, so they don't have a baseline of what looks "natural." However, since I am doing the move in a way that is probably similar to how they would too—since I'm "mirroring" their instinctive actions without them ever even doing it—then I think it will inherently feel fair to them. It will draw no attention to itself in the way a more dextrous handling of the cards might.
Again, this is all personal preference. My goal is to seem as "normal" with a deck of cards as possible. If you're performing professionally or you want to come off as a master card handler, this is not a concern of yours. While my interest in magic is clear to my friends and family, I try to never come off as someone who is practicing moves and working on standard sleight-of-hand skills. Generally, I want to imply my interest manifests itself in doing research into arcane ideas and trying stuff out with real people, not locking myself away and assiduously rehearsing moves in front of the mirror. So, for me, a double turnover that mimics the way a regular person would do that action without thinking is a good thing to have.