Any strike of sufficient power to the head can cause unconsciousness, but not for twenty minutes. That is a very long time to be out and probably indicates a very serious injury. Most strikes cause a K.O. not from sheer damage but through shock value: the head takes a hit and rebounds off the skull, causing shock to the brain, which "reboots" after the shock. Such a hit usually results in a very big headache and a disoriented fellow waking up anywhere from a few seconds to several minutes later, but not likely twenty minutes. The fellow also does not wake up fully functional and will suffer a variety of ill-affects for some time.
A particularly common strike known to cause instant K.O. is a punch to the side of the jaw. But the fellow will only be out for a minute or two at most, and more often only fifteen seconds.
The forehead is the most reinforced part of the skull, and thus the least likely target for a K.O. The temples on either side can be used, and the back of the skull is the weakest and therefore best target to injure someone.
Striking the ears can result in disabling a person instantly, though not usually from K.O. The inner ear handles balance. Striking both ears simultaneously will cause the person to drop like a rock and be disoriented for several minutes. It can cause intense nausea and vomiting as well. Striking just one ear is less effective but still powerful. Again, however, this will not last a full twenty minutes.
The chokes that work best in combat are not air chokes. A person can stay awake in an air choke for upwards of two minutes. A blood choke cuts off blood supply to the brain and causes unconsciousness within seconds. Blood chokes are something only trained combatants use, however. A person on the wrong side of a blood choke will wake up within a minute or so unless the choke is maintained after he falls unconscious. The amount of damage done then is dependent upon the duration of the choke, up to permanent brain damage or death.
If you just want the person disabled for a long time, shatter the knee. A kick targeting a straight knee will force the knee back. If the knee can't go back (you are bending it the wrong direction, after all), it will break. This is incredibly painful and that alone will immobilize most people, either through pain or the resultant shock. Hardier folks still won't be able to use the leg, and it will probably never heal fully. (This is one reason why fighters lower their stance and bend both knees.)
Hitting the side of the knee can also break it, whether the knee is straight or bent. It requires more training to do, particularly if the target is facing you head-on, but it is certainly possible. This type of break won't be as debilitating (usually a break, not a shatter) but is nearly as disabling.
I could give more ideas, but this is already kinda long, so...