I've been trying to find this out (especially about kīlaka for over 20 years.  

1. Thomas Coburn in his book "Encountering The Goddess" note 27 to Chapter 6 says:

Ambika Datta UpAdhyAya observed that all mantras, Rg Vedic and other, have placed over them a curse which renders their recitation ineffective until it is removed with a kIlaka, a sort of "counter curse". An appropriate kIlaka exists for every mantra.  The episode that appears here in the kIlaka is clearly similar, but the restraint is placed on the mantra in SaptazatI not as a curse, but to contain its overwhelming power.

2. The TAK entry for kIlaka is:

Au sens figuré, le kīlaka est un des éléments servant à caractériser

un mantra tantrique et qui, dans les rites, doivent être imposés,

par nyāsa*, en commençant par le ṛṣi (v. s.v. rṣādi). Ce

kīlaka est conçu comme la pointe grâce à laquelle le mantra va

pouvoir se ficher dans la personne ou l’objet qu’il vise et donc

agir. Les manuels de rituel qui mentionnent le kīlaka le donnent

comme formé par un des éléments constitutifs (mot ou syllabe) du

mantra. Une telle mention ne paraît pas être très ancienne. [A.P.]

Le kīlaka, PHAṬ􀄝*, du sudarśanasahasranāmastotramahāmantra est

mentionné dans le Sudarśanasahasranāmastotra donné en appendice

de l’AhS (p. 617). [M.R.] Voir aussi TBhS, p. 170, citant un

texte non daté. La mention du kīlaka est courante à l’époque moderne.

[A.P.]

3. Other than whats above.  The vague contradictory information that I've  received 15 0r 20 years (I no longer remember the sources)  is that it is like a bolt of a lock or a door.  Some informants saying that repeating the kIlaka bija syllable removes the pin of the lock or door releasing the power of the mantra.  Other informants saying repeating the kIlaka bija syllable puts the pin in the lock or door of the mantra to contain its power so the repeater of the mantra isn't destroyed by the power of the mantra.

If you receive any information offlist please pass it on to me.

Thanks,

Harry Spier