Kilakam literally means the "lock" or the "pin".. So, before chanting one needs to un-lock the Mantra by performing the NyAsa ritual as prescribed. This is said to release the full potential of the Mantra.
We have the following verse in KulArnava Tantram's 4th chapter:
Rishirashya parah shambushcha chandah avyAtkapurvikA |
devatA chAtra sarvamantreswari parA ||
... shadangAni cha pArvati ||
The commentator elaborates on the word "ShadangAni" as follows:
ShadangAni -- All Tantrik Mantras have 6 limbs (Shad+Anga) -- Rishi (seer),
Chandah (metre), DevatA (Deity), Bijam (seed), Shakti (energy) and Kilakam (pin) --- and their respective
NyAsa places are --- the top of the head (Shirah), face (Mukham),
heart (Hridaya), anus (Guhyadesha), feet (Padadvaya) and all body
So, as per this commentary, the Kilaka NyAsa is to be done in the anus. But this is not universally followed.
Kilaka NyAsa is more widely done in the navel for example.
So, it depends on the SampradAya, the particular Mantra etc. When, the Guru gives you the Mantra he will tell you where exactly to do these ShodAnga NyAsas on.
We do this NyAsa, with the right hand in a particular Mudra, while chanting the corresponding Viniyogah of the Mantra.