It works fairly well. Key here is that it only works with lights that use an even number of batteries because you have to invert exactly half of them for this trick to work.

It also is important that the batteries all be of the same type and relative strength, age, amount of energy remaining.

This was far more of an important insiders trick when flashlights generally had switches that were easy to bump into the on position. Most modern flashlights have rotary or recessed push-button switches that are pretty darn hard to activate accidentally.

Packing for critical situations where the packed flashlight is going to be banged up in transit I leave the batteries out. I also tape over the ends of the batteries with tape and keep them in a bag to eliminate any risk of them shorting out.

This is a trade-off. In some ways with a high-quality flashlight there is less risk of the flashlight accidentally turning on than of the batteries being lost, damaged or shorted out by moisture, sweat and salt water are the worse, or conductive gear.

Inverting half of the cells would give you an additional level of protection. But so would taping the switch off or inserting a non-conducting disk that could be removed when you need the light to work. Off and on I have inverted half the batteries but one day when reinstalling them in a four cell light it dawned on me that in a desperate situation and less than perfect conditions it would be easy to drop the batteries and have to hunt them down at the worse possible time.

I can see this quite clearly. I'm cold, wet and standing in a puddle up to my ankles during a freezing rain at night when I drop one or more of those batteries in the murky water. Screwed in a situation of my own making. Unhappy camper.

Be careful about that insulating disk also. I have never used this method but I could picture that bit of cardboard falling down into the flashlight where you can't get to it easily. Given time, some light and a handy stick you could remove the offending bit but this would be a hell of a job to have to do under pressure.

Years ago I got carried away taping the spare batteries and it took an act of congress, and a sharp knife, to extricate the batteries. There was no rush at the time but since then I try to make sure the tape can be removed quickly without need of a tool.

I don't know about all LED flashlights but all of the ones I have used show no ill effects from reversed polarity. The diode blocks the voltage. No harm done. But be sure to check your light. Murphy's law says that those batteries will be inserted the wrong way in the heat of the moment. You want to know how your light or device will react before your in the middle of an emergency so you can leave vulnerable devices at home where such weakness is more acceptable.

I wouldn't keep a device around that was so sensitive. I would give it away or otherwise pension it off. Even at home if I can't count on it I don't want it around.