I asked this question in another thread and someone said it's a variation of 'ti' used almost exclusively in the North:
ti vs. chi
EDIT: Say Something in Welsh just mentioned this in the intermediate course, it seems to be used in places where you'd use 'fo'/'fi', ie. after a vowel. Efo chdi, i chdi, etc. I must say that it feels really nice to say, especially after 'efo'.