These phones need a kill expression or finger. If you touch a sensor with your left pinky or wink at the camera it nukes the phone.
A solution that can seem like plausible deniability could be interesting.
If the phone is in your pocket and somebody puts a gun to your head and tells you not to move, you are not pressing anything on your phone.
My impression is deliberately doing this would be illegal. It would have to be convincingly deniable somehow.
Is there a way to do that?
You'd also have to rely on this unnamed other to force that particular finger, rather than the others...
suspect: "no you cant force me to put my pinky there", attempts to make pinky inaccessible.
other: "we will charge you with obstruction if you resist placing the pinky"
E.x. if one had a "dead man's switch" phone that required a passkey every x minutes, and each time you did so it set the next threshold...
Which requires them to prove they know that device likely contains relevant information. Just being party to a court case doesn't mean you're forbidden from deleting anything ever again... like I said there are very specific rules for evidence, and one cannot begin to claim something relevant is destroyed if you can't even show that you had any idea what might have been destroyed in the first place.
You're right that in normal circumstances you can routinely delete records for data hygiene, to save money, as part of a phone repair, and so on, unless you've been court ordered otherwise.
And remember that without a court case alleging something in the first place, they wouldn't even have access to the device to know 1. it existed and 2. it might have had something useful on it. If I had two devices in my house and they're both clean, you can't just say "oh we think one of them had some evidence that was destroyed"... you need some kind of proof that it at least likely contained something relevant in the past before you can even begin to presume it might have been destroyed.
Same for your second paragraph: "oh we think one of them had some evidence..." - that's not how it works! It's your intent to destroy evidence that is the crime, not whether you destroyed evidence. They do not need to prove you destroyed evidence or even likely storage of evidence to get you convicted.
This is the main thing you're saying that is bad legal advice.
For your other point, yes, if they can't tell anything happened, or it seems like an accident, then you're probably going to get away with it. This happens a lot. I think that's a different topic. Original topic was: if you wink at your phone or use a weird finger (or some other visible gesture) and now your phone's wiped, could you get in legal trouble for that sequence of events.
Accidentally destroying evidence can still carry a serious penalty, but yes the intent is generally the most important. But absent intent, it can still help the prosecution to know the device had something useful to them on it.