Generally, if you want to pass food to someone, put it on a plate so they can pick it up themselves.
The only reason to use the back of the chopsticks, is if there is a shared plate of food in the center without a separate set of serving chopsticks. Taking from the shared plate with chopsticks that have been in your mouth could be considered unhygienic. You can use the back of the chopsticks to move the food to your own plate, then eat it.
However this is more like advanced etiquette and not a crucial rule, in my opinion. The only really bad things to avoid are sticking your chopsticks upright into rice and passing food between chopsticks.
Generally, if you want to pass food to someone, put it on a plate so they can pick it up themselves.
The only reason to use the back of the chopsticks, is if there is a shared plate of food in the center without a separate set of serving chopsticks. Taking from the shared plate with chopsticks that have been in your mouth could be considered unhygienic. You can use the back of the chopsticks to move the food to your own plate, then eat it.
However this is more like advanced etiquette and not a crucial rule, in my opinion. The only really bad things to avoid are sticking your chopsticks upright into rice and passing food between chopsticks.