You can be under arrest before you are handcuffed and the police officially read you your rights. The problem from the citizen perspective is that there is no bright line test that tells you whether you are under arrest, and the courts can find that there was no arrest at a point where the suspect was handcuffed and confined to the back of a police car (United States v. Bullock, 632 F.3d 1004) – you can be merely "detained" for an investigatory stop while in cuffs. One test that might be used is asking if you are free to go about your business. A reasonable person could decide whether they were under arrest by the nature of what the officer says. For example, if he says "It would help us if you could stand over there" or "...if you could sit down", it is reasonable to conclude that this is an urging but not a command. On the other hand, if he says "Sit down, now!", it is unreasonable to think that that is a mere suggestion or plea, it is an order.
There is no specific Arizona law that says when police can order you to do something, nor is there a specific law saying that force may only be used in such-and-such circumstance. There generally are guidelines for police conduct, and the guidelines tend to grant much leeway to officers (until it gets to be a recurring problem and the guidelines are changed). An example is Seattle, whose police manual reduces the question to the statement that "An officer shall use only the force reasonable, necessary, and proportionate to effectively bring an incident or person under control, while protecting the lives of the officer or others". But this does not say whether one must obey police orders.
In Oregon v. Ruggles the court sympathetically notes that
Whether a particular police order is “lawful” is frequently a complex
question involving some of the most vexing and intractable issues in
constitutional law. For example, a police order such as “Stop!” can
be an unlawful seizure of a person under Article I, section 9, of the
Oregon Constitution, depending on whether the order is accompanied by
a sufficient show of authority and the officer who issues the order is
subsequently found to have lacked reasonable suspicion to believe that
criminal activity was afoot.
But Oregon has a statute, ORS 162.247(1)(b), requiring you to obey a lawful order by police. The court found that you don't have to know whether the order is lawful. Arizona has a related statute, ARS 13-2508 where resisting is defined as
intentionally preventing or attempting to prevent a person reasonably
known to him to be a peace officer, acting under color of such peace
officer's official authority, from effecting an arrest
including the means of "Engaging in passive resistance" (which is "a nonviolent physical act or failure to act that is intended to impede, hinder or delay the effecting of an arrest"). Arizona law frames the resisting crime in terms of "effecting arrest", which is different from what Oregon law says:
(a) Intentionally acts in a manner that prevents, or attempts to
prevent, a peace officer or parole and probation officer from
performing the lawful duties of the officer with regards to another
(b) Refuses to obey a lawful order by the peace officer or parole and
But even if an individual in Arizona is not chargeable with resisting arrest for failing to sit (because the officers were not effecting an arrest), that does not mean that police cannot order you to sit – it just means that it's not a separate crime to fail to comply.