How does Mirai's ring help her control her ability? The page on Kuriyama Mirai on Kyoukai no Kanata Wiki says she uses her ring to activate her ability.
From the show, I got the feeling the ring was something that kept some of her powers on check, rather than an activator.
I don't recall any explanation of how the ring works during the series.

Whichever the case may be, how exactly does the ring help her control/activate her ability?
Does the ring possess some kind of magic? Or is it some kind of psychological power it exerts over Mirai?
This answer only applies to the TV series proper. Additional information is given in Mirai-hen; see Amir's answer. 



There isn't really much to be said about the exact function of her ring. It isn't discussed in the novel (though she is shown wearing the ring, cf. p004-005 of LN 1), so the best I have as far as a textual citation is this magazine scan (SFW image; NSFW site), which reads:


  自分の血を剣に変えて戦う未来。普段は包帯と小指の指輪でその能力を抑えている
  [...] For the most part, she wears a bandage as well as a ring on her little finger in order to keep her powers in check.


There isn't really any verbalized explanation of how the ring works in the show either - the best we can do is infer its function from its depiction in the show. 

So, basically, it's probably just what you thought it was - a power limiter of sorts.