If a character wishes to disable their opponent’s arm by stabbing them with a knife from behind, which would deal more damage – leaving the knife in, or pulling it out?

Pulling it out.

If you leave a knife in the body, then it blocks the blood from leaving the body. As the attacker, this character will want their opponent to start bleeding out and to keep their weapon. It is worth noting that a knife is a solid stabby weapon that’s very good at killing.

This is why characters who dramatically pull the knife out in media are morons. They are in a highly stressful situation, engaging in strenuous physical activity, both of which will lead them to a quick bleed out. (Your heart pumps your blood through your body and it will not stop just because someone cut you. The faster your heart goes, the more quickly blood is being pumped. The blood must flow somewhere. In this case, right out the hole.)