I am looking for a flashlight with a simple on/off button and the modes (low, medium, high) being controlled by turning the flashlight “head”
so that the flashlight remembers the selected mode. Is there a technical name that describes the mode selection by turning the flashlight head ? Can anybody recommend a brand ?
My Eagletac T200C2 has this feature. I’m not sure what the official name is, but I call it a twisty.
Those tactical head-twisters are made, for example, by Eagletac. The head (or module in T25C2) has power contact with non-anodized thread on the body, and two control contacts - pins and circumferential contact on the bottom PCB. When only the power contact is engaged (loose state), you have low, half-tighten engages the pins (med), full tighten - high.
the acebeam k40 uses a magnetic ring switch
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Eagletac T200C2 looks good! Not really budget friendly unfortunately.
Armytek Predator pro v 2.5 has this. My favorite UI ever, and one of my favorite lights. You might be able to pick up used for a good price. I would assume the newest generation predators also operate like this.
I have this flashlight in my wishlist on AE for a long time.
Because of the mode switching ring on it.
problem i that other budget lights I have with Yupard on it do not have a quality feel.
I like the silver color though and every once in a while the price drops to $16 and I am temped to buy it.
this one looks sturdy
For lovers of the twisty UI.
The EagleTac T25C2 modules are not that expensive for buck drivers with 2.7V-12V range.
The single led modules, covering everything from UV to IR, Nichia 219 included, start at $24: http://www.illuminationgear.com/EagleTac-T25C2-LED-MODULE-ETT25C2MODULE.htm
The triplets with TIR start at $36: http://www.illuminationgear.com/EagleTac-T25C2-MULTI-LED-MODULE-ETT25C2MULTIMODULE.htm
The only thing you need is a host capable to accept such modules and properly control them. Such host can be devised from C8 by redesign of the head and fore part of the body, adding an extension tube for additional 18650 and two plastic sleeves for use with CR123a. The switch can be replaced by forward clicky. The current C8 host costs $12.25, so the redesigned and extended version should cost no more than $25, provided that sufficient number of orders will be collected.
The Olight M2X-UT has a tail-switch with momentary and permanent on/off, as well as quick-turbo and strobe. There are three power-modes that are controlled by means of twisting the head.
The Yupard is $14.80 if bought through the Aliexpress app & the other light $12.79 if bought the same way 
Ah I ordered the sturdy looking one. All my other light are rated a 2meters deep water and our pool is 2,20 at the deepest part so I absolutely need this right, right? (Well at least the tremblings have stopped now :-P)
Lol
Yes that extra 20cm of water could make all the difference 
A lot of dual-switch flashlights work in a similar manner. On-off controlled by tail switch. Mode controlled by side button.
Like you, I like to feel confident in the flashlight turning on in the same mode. (And not having to worry about whether I accidentally switched modes by clicking my flashlight on-and-off too quickly.)
Examples: Yezl Y3, Shadow JM35 (2x26650 only)
EDIT: Also Eagle Eye X5R, on sale now: “[link]”:Gearbest New Arrival: March 22nd, Convoy H1 CREE 29.99$ - #328 by gearbest