Ref # A Dictionary of Intermediate Japanese Grammar p333
This is a rare case of 一級 grammar that I hear in every day conversation. I learnt this when I was still a beginner. I think it has a similar meaning to "〜たまま".
It is a colloqual form of 放し, meaning doing something and leave it at that.It has a lot more emphasis than ...たままにする.
How can I know how must I read the kanji, if in on yomi or kunyomi. For example: in nﾂｰ 5446 is: koko ni "TE" ki ppanachi or koko ni "OKU" ki ppanachi? It is very difficult for people who donﾂｴt know japanese very well.
It's not that difficult. The fact that there's a hiragana 'ki' after it tells you that it should be 'oki'. This has nothing to do with this grammar though...
It's common practice that in compounds you'll be using onyomi.
For Futaro - this link gives a good description of what is going on with the on'yumi/kun'yumi issue. http://www.tofugu.com/2010/03/23/the-types-of-kanji-in-japanese-onyomi-vs-kunyomi/