If you have a chance to meet a native person of Malang and ask, “Why do you pronounce polisi (police) as silup rather than isilop? You said that it is Boso Walikan,” the person may have just answered, “It is just there.” Boso Walikan or Reversed Language may not have strict grammatical rules. Either way, the way we reverse a word can be various. It can be acceptable when we reverse arek (young people) to kera. On the contrary, it can be quite strange when we reverse polisi to silup but, somewhat, it is still accepted. There is no other way than to keep puzzled with Boso Walikan.
Social interaction has a huge influence in the growth of Boso Walikan. You can learn more about Boso Walikan by having Kamus Bahasa Malangan (dictionary of Malang language). As a slang language, how one word can mean something different may have no exact reasons. This is about the uniqueness of human language itself. The grammatical rules of a slang language can be said as largely arbitrary. It was born from the social interaction. When one word is repeated from time to time, a new meaning can be born as well.
There are at least two main rules of using Boso Walikan: easy to pronounce and acceptable. Similar to other slang languages, Boso Walikan is also unusual and startling. It is commonly used in an informal conversation. You can learn Boso Walikan by making a conversation with native people from Malang. That is the easiest and most practical way.