I shall try.
The Mosaic law came directly from God, and was a covenant between physical Israel and Jehovah. Jesus fullfilled that law, and so ended it. The only parts they hold as still valid are parts repeated by Jesus as still in force, or parts that came before the law. For instance, Noah was given the prohibition on eating animal blood long before the Mosaic law was written, and the blood thing was restated in the Greek Scriptures---so they keep it.
As far as principles, this is where they get to be creative. Tattoos for instance. Christians are not prohibited from getting them, because they are not under the law. However, the law against tattoos was to separate the Israelites from the nations around them. So the principle is still a good one---do you want to look like the world? How would that be viewed by people that know you are a Christian? What about the most important thing you do---the ministry---wouldn't that look bad? So while we are not under law when it comes to tattoos, the principlel is still valid.
*I stated the above in a positive manner---it's all bullshit to me and should not be understood to mean that I agree with any of it.
Hope that helps.
NC