Odd question here. If Ekans bit its own tail, how hard would it have to pull in order to rip itself in half?
Also relevant. http://imgur.com/gallery/l7W54
No, see, it's quite possible. In order to pull back on the tail, Ekans would first have to have leverage. and in fact, it has great leverage: its own tail. Assuming its jaws do not come undone, it is exerting double its own force by pulling back on the tail, so in theory it would only require half the force to rip itself in half, since it is pulling back with its own tail just as hard as it is with its mouth.
Ekans wouldn't be able to do a bit more than nibbling its own tail. If it extends its mid-section maybe it may be able to pull its tail a little.
It would be as difficult as a human doing the same by biting its own feet.