I've seen this thing first time in my new room. Dual flush tank (see image 2, it's not an actual photo, but where I live has similar thing as this one).
Usually, I saw a flush with either a liver or a single button at top that you pull down / push once and release. The flush works fine. (image 1)
Now, in image 2, you see a dual flush button. Now I searched a lot on Google. Everyone told you have to press one of the buttons depending on the need of amount of water. But none of them told whether you have to press and hold or just press and release.
Of course, finally I verified it myself. When I just pressed one of the button and released it at same time, very little water came (insufficient). I thought something is wrong as I'm used to the flush shown in image 1. But then suddenly it came up to mind. I pressed and didn't release it. I got what I wanted - sufficient water, it kept flushing until the water emptied!
The question is: Are dual flush made to press and hold (not release until wasted is flushed fully)? Or is it something wrong with the flush I'm using? Further, can both buttons be pressed simultaneously for full flush?