while charging or After charging it should be Used

It's a feature you use after charging when the phone battery is low.

Making it last longer before it turns off because of not enough charge.

It turns off some features to conserve power

Use Low Power Mode to save battery life on your iPhone - Apple Support

After reading through the limitations of using Low Power Mode, use it whenever you wish.

darshan2611 Said:

"When Should we use Low power mode Features In our Iphone "

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The Answer:

When you have less of a chance to make optimum use of it, such as having no place to change it.