Let me explain this in a way you'll be able to understand:
Let's say you're a nameless shinobi & your mission is to stick your kunai into Obito. When you get into stabbing range, rather than physically fight with you, Obito just warps you into his pocket dimension. So, not only do you fail to stab Obito, but he doesn't even bother to fight with you.
Then it gets worse, Obito let's you out of the pocket dimension a few time per week to ask your opinion on his shoes or your thoughts on X Factor, which you don't even watch, & you begin to think that maybe he'll slip up & you'll still get a chance to complete your mission.
So Obito constantly lets you out to ask you stupid things, but when you go to grab your kunai to stab him, he senses it & just puts you back...eventually this makes you so frustrated that you stab yourself with your own kunai repeatedly.
-Hope this helps