What is rating's change based on? Ratings at the beginnineg of the game or the end? For example assuming that two players A and B start a game where A's rating is 1300 and B's rating is 1400, but each one had simulateously played other games and had changes in their ratings so just before the end of their game (A vs. B) A's rating is 1380 and B's rating is 1350. which couple of ratings is considered to calculate the change between A and B? is it (1300 ; 1400) or (1380 ; 1350)?

Cheers

They are done at the end of the game, so, in your example above, they would be based on 1380 and 1350. Hope that helps.

I figured that out myself, but do you care to explain the reason behind this? Ican imagine that people think they are cheated out of some points this way. Isn't it normal that you play a game, and the ratings with what you start are how it's changed?

Depends whether you are A (1380) or B (1350). A gets less ratings points for a win and B gets more. It all works out even after a number of games where players switch continuously between A and B.

You cannot get cheated out of ratings points. They can be inaccurate or ... inaccurate.