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.