From the Star Wars: Visual Dictionary and Force Awakens: Visual Dictionary .
The "on button" is the big shiny thing on the handle.
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Straightforward answer:
Yes , as Pablo Hidalgo of the Lucasfilm Story Group recently said here and here
, it's a mistake and that the lightsaber is still supposed to activate with the plate in the middle.
Q.
Why in the Force Awakens does Finn clearly press the blade adjustment button on Luke's lightsaber to turn it on?
Is it Snoke? ‏@pablohidalgo -
It's a mistake.
Via Twitter and Q.
Will Anakin's lightsaber ignition button be corrected in futuremovies to fit how it was in the ot [Original Trilogy]?
‏@pablohidalgo - If you're asking if the same mistake made in TFA will happen again, I'd say I hope not.
Via Twitter
(with grammar corrected) One assumes the same applies to the TCW episode.
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Let's say it has both the activation button and the length adjust control as depicted in the dictionaries.
Presumably the activation button would cause the blade to quickly jump up to the set blade length.
However, if you left it on, but turned the length to 0, maybe holding the length adjust button down would cause it to extend in a slow, cool, way!
That way, both the OP images could make sense.
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In episode 2 of the unfinished episodes of the Clone Wars, which I believe are canon, Anakin turns his lightsaber on by pushing the big red button in the front.
This, added to the many ways you cited by which this lightsaber can be turned on, leads me to believe that there is in fact more than one way to turn it on.
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It's like a TV remote - I can turn on the TV by pressing the red 'standby' button, or by pressing a channel number.
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I would say it works like a dimmer switch.
One switch just controls off and on while the other can control blade length and intensity while also controlling the on off capabilities.
Redundancies built into the weapon to make it more useful.
This also explains why Darth Vader comes out at different speeds.
Sometimes he uses the blade length adjustment knob sometimes he uses the on off switch
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You can tell from  this video  that Finn is actually pushing that button to activate the lightsaber       It seems like there are different on-buttons or something because the toys say the big rectangular one near the middle is the on-button.   From  this video :    Is there more than one on-button? Or is this just a continuity error?  Where is the official on-button, and    why the seeming inconsistency?
Which button activates Skywalker's lightsaber, and why is it inconsistent?
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<star-wars><technology><lightsaber>
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It's possibly that this is a simple mistake and continuity error. However, another possible explanation is that one button is an "on" button whilst another is a "blade length" button that also functions as an "on" button. This way, it would be possible to turn on a saber with both buttons. Opinions differ but the most authoritative view (from the film-makers themselves) is that the scene where an alternative button is used to switch on a lightsabre is a mistake and the actual on off button is the large red one on the handle.
The big shiny thing on the handle is the on button. When Finn presses the adjustment button to turn a lightsaber on in The Force Awakens it is a mistake according to Pablo Hidalgo. Both the activation button and the length adjust control are required to switch it on. There are many ways to switch on a lightsaber - as there may be many ways to switch on a TV remote. It works like a dimmer switch, with different controls for on and off, for blade length and for intensity.