Parental alienation syndrome is a term often used to describe what happens when children are discouraged from closeness with one parent by their other parent, usually without good cause.  As such, it has been abused by many parents who were justly alienated from their children, often because of abuse, either of their former spouses, or of the children.  Unfortunately, this leaves the legal system reeling, because they are caught in the middle, usually not knowing who is the one making false accusations, and who is actually telling the truth.  They tend to err on the side of trying to keep both parents in the children's lives by thinking, often erroneously, that the father is most likely to allow access by the mother, rather than the other way around.  As a result, many formerly abusive fathers begin to engage in what is popularly becoming known as maternal alienation.  
One of the first mistakes that is made in alienation cases is to believe that a father who wants custody of his children must be a good father, because most fathers would not want to be involved.  Unfortunately, many of the fathers that show the most interest in getting involved with their children upon divorce are those that were previously either uninvolved, or only minimally involved with their children and do get involved, and pretend to be model fathers only to exact revenge on their ex-wives.  Naturally, we hear of a few of the most extreme cases here and there.  Unfortunately, the ones we hear about have the most unhappy endings, of either the children, or the wife getting killed.  Many of these men look at their children and their wives as possessions rather than people, and if there is a threat of having their possessions "stolen" from them, they will act irrationally.  However, the cases of anyone getting killed are very rare.  Mostly, it just happens that the father will alienate the children from their mother, often making false allegations against her, and stating that her claims of abuse are false, and he is innocent of any wrongdoing.  He will brainwash the children until they remember nothing of their former lives and he seems completely innocent before moving on to the next victim, whom he may even groom to abuse his former victim for him.  Looking at the traits of men who alienate women from their children, they seem to display characteristics that are commonly shared by sociopaths, and may indeed even be sociopaths.
Unlike the parental alienation that has been debunked, time and time again, maternal alienation is not a syndrome, but rather an action by a bitter, often sociopathic man who wants revenge on his wife or lover.  A man who is engaging in maternal alienation is likely to take every mistake that has ever been made by the mother of his children, and turn it against her, as if one time mistakes were actually a pattern of behavior.  He will threaten and intimidate her if possible into giving up the children, telling her that she cannot win, or else running her into the poorhouse so that she can no longer afford to care for the children.  He will often use friends and aquaintances against her, and even if he cannot convince others to dislike her, which he often does by playing the victim, he will convince her that they dislike and distrust her.  It is a form of psychological warfare on the part of the alienating parent.  He will tell the children stories about how they were unwanted and unloved by her, essentially brainwashing them into compliance with him.  If that doesn't work, he will often browbeat and make the children feel as if loving her is a betrayal to him.  
When he finds a new partner, he will invite her into the fray.  He will play the victim so well that she believes it is indeed his ex-wife who has committed all the acts that he has done himself.  He will already have the children brainwashed enough by the time she comes into his life that the only person insisting that his ex-wife is not this ogre to be feared is his ex-wife, and she is grossly outnumbered by this point.  He will have already discarded any friends who may have seen or noticed the truth, so that his new wife will have no chance of finding out the truth.  How long he keeps up the facade is dependent on how long it takes to lull his new partner into a sense of complacency and dependance.  She in turn will continue to abuse the ex-wife on the grounds that she is defending her new partner, who must be the most wonderful man on earth to have taken his children and raised them so kindly.  He plays the long-suffering, loving father role to the hilt, even though he seethes each time his ex manages to slip it by him to her children that she loves them.
This is a mother's view of what maternal alienation is.  This does not deal with the courts or anything else, but simply tells the facts about a much misunderstood and maligned problem.  The fact is that until people understand that there is a problem, they cannot solve it.  Alienating children from mothers who love them is a problem in today's society, and will continue to be until people start to recognize it.  This is not to say that mothers should automatically be given custody of their children, but is simply to get people to start asking why children need to be removed from the custody of mothers who are not having other problems.  If more judges asked this question, instead of simply assuming that fathers would not ask for custody unless there was a problem, the family courts may be a much better place for all.