Those are your two key questions.
These guys aren't advising you to be a jerk, but you should know what your rights are and how they apply to you.
Being detained means they may put you in cuffs for their own safety while they try to understand the situation. (Let's say you are involved in a domestic squabble.) You are temporarily under the state's custody but you aren't being charged with anything.
You know what being arrested means. You don't want that.
If you are not under arrest then you don't have to say anything. If you are, then what you are using is your right to remain silent ala miranda.
"No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury, except in cases arising in the land or naval forces, or in the Militia, when in actual service in time of War or public danger; nor shall any person be subject for the same offense to be twice put in jeopardy of life or limb; nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation."
The relevant bit here is the part about being a witness against yourself. You can't remain silent when asked for identification, but you don't have to answer the cop when he asks you if you know how fast you were going. In that situation, you're pleading your 5th amendment right.
1. If I know I am totally innocent, I am totally honest with the cops 2. If you are kinda guilty, i.e., you did something wrong but it wasn't that bad, supplicate yourself to the cop and place yourself at their mercy 3. If you are guilty and know it, shut the fuck up
One thing you have to be careful about: you may end up doing yourself more damage by pleading the fifth when it isn't necessary.
"No officer, I understand this is a routine traffic stop, but I'm pulling out the fifth and am not responding to your queries."