The specifics of every PoS algorithm are unique, but as a general rule, PoS algorithms require you to take "turns", and the more you stake, the more validation "turns" you get. If you try to validate when it is not your "turn", you are A) ignored. and B) will lose your stake if it is done fraudulently. There is no need for a difficulty level in PoS, because everyone is only allowed to participate up to the amount of money they invest. When it is your "turn", and a sufficient interval has passed from the last block, you get to validate.
"Turn" is defined loosely here, as there are usually multiple validators and complex algorithms to stipulate when you are allowed to validate. But a turn is a good metaphor for what happens.
Thanks for your reply! I think for a newbie account your comment is definitely worth a merit, so I'm giving you one. But, since you know the mechanism, can you please elaborate a bit more on that "sufficient interval" between blocks? Since there are no time consuming calculations, as we have in PoW algorithm, how the "delay", if I can put it that way, before appearance of a new block is achieved in PoS?