(Late) Shri H R Seshadri Iyer,  a great astrological scholar says that Kaala Sarpa Dosham is not properly understood by many.  Generally if all the planets are sandwiched between Rahu and Ketu it is termed as Kaala Sarpa Dosham (KSD).  This is not always correct. 

He says Rahu is a cobra and poison in the mouth while Ketu a Scorpion has it in his tail.   The movements of these being anti clockwise, planet in between the mouth of Rahu and the tail of Ketu only get hurt.  It is here that KSD exists and not when they are between the head of Ketu and the trunk of Rahu --- for elementary reason that ---- scorpion cannot hurt by its mouth and cobra by its tail. 

To put it more clear  -- if all the planets are from Ketu to Rahu in clockwise order the Dosha is formed,  and not,  when in the space of Anticlockwise order (that is from Rahu to Ketu clockwise).