Rear Adm. John M. Poindexter did more than answer a crucial question about President Reagan yesterday when he testified that he didn't tell Reagan about the Iran arms sale funds diversion. He drew a portrait of Reagan that bore little resemblance to the disengaged, uninformed president with a lax "management style" presented to the American people in the Tower special review board report last February.

The picture that emerges from Poindexter's critical testimony before the Iran-contra investigating committees is of a strong-willed president who knew exactly what he wanted and was willing to take great risks to achieve it -- so great that Poindexter, as Reagan's national security adviser, thought he had to take steps to protect Reagan from himself.

And two of Reagan's most important goals, Poindexter said in his methodical, careful, low-keyed style of giving testimony, were to get back American hostages being held in Lebanon and to support the Nicaraguan contras at all costs.

He said the president had been under "no illusions" about the stakes involved in a swap of arms for hostages, and remembers Reagan remarking: "If we get all the hostages out, we'll be heroes. If we don't, there will be problems."

Similarly, he remembered the president telling him aboard Air Force One as they flew back from an economic summit in Toyko in May 1986, "Look, I don't want to pull out our support for the contras for any reason. This would be an unacceptable option. Isn't there something that I can do unilaterally?"

In Poindexter's account, Reagan also is a president who set the tone among his top advisers for the climate of secrecy that flourished inside his White House. "The president's feeling was that the way you carry out a secret, covert activity is that you limit the knowledge to the absolute minimum number of people," the admiral said, slowly puffing his pipe. "There were discussions about that."

He also said that some of the president's top advisers "spoke in favor of withholding {congressional} notification {on arms sales to Iran} as long as possible, and the president clearly agreed with that."

Poindexter's decision not to brief Reagan on the arms sale funds diversion to the contras, as he explained it, grew out of two convictions: that Poindexter understood the president's mind and motivations, and that he believed Reagan had granted him broad authority to implement the policies the president most wanted carried out.

Referring to the diversion, he said: "I felt that I had the authority to approve it . . . . My role was to make sure that his policies were implemented. In this case, the policy was very clear, and that was to support the contras. After working with the president for 5 1/2 years, the last three of which were very close, probably closer than any other officer in the White House except the chief of staff, I was convinced that I understood the president's thinking on this, and that if I had taken it to him, that he would have approved it."

Thus, he testified:

"I made a very deliberate decision not to ask the president so that I could insulate him from the decision and provide some future deniability for the president if it ever leaked out."

Poindexter's portrayal of Reagan is at sharp variance with that of the assessment of the Tower review board, which investigated the policy-making process at the National Security Council that led to the U.S.-Iran arms sales and diversion of profits to the contras fighting the government of Nicaragua.

"We were all appalled by the absence of the kind of alertness and vigilance to his job and to these policies that one expects of a president," said former secretary of state Edmund S. Muskie, a commission member, after the report was made public.

That's not the Reagan that Poindexter described in his first day of testimony. That Reagan is a president who reads everything put before him, who listens carefully to advice, weighs differing views and makes his decision with full understanding of the risks involved.

Poindexter offered that assessment based on having "observed and worked with the president for 4 1/2 years before becoming the national security adviser, so I felt that I did have some advantage there."

In response to questions from Senate select committee counsel Arthur L. Liman, Poindexter testified that he daily gave the president papers dealing with issues that he "had a keen interest in."

"And is it fair to say," Liman asked, "based on your experience, that the president of the United States is a reader?"

"Yes, he reads everything you give to him," Poindexter replied.

He also said Reagan had a tendency to sign off quickly on something he wanted done, including the signing of a "finding" that approved shipments of arms to Iran for release of American hostages that had already taken place. This was the Dec. 5, 1985, finding that the admiral testified he later destroyed to save the president "significant political embarrassment."

In his most revealing account of Reagan as decision-maker, Poindexter described a "free-wheeling discussion" meeting with National Security Council members in the White House family quarters two days after that first arms deal was signed. The subject was the swap of arms for hostages. "And the first objective was to get the hostages back," Poindexter recalled. "And the president felt that . . . it was worth taking some risk."

The debate was long, with Secretary of State George P. Shultz and Defense Secretary Caspar W. Weinberger registering "very strong, specific objection and clearly laid out for the president the other side of the issue."

Liman asked him: "And there's no doubt in your mind that the president listened to and understood those objections?"

Poindexter replied, "I have a very vivid recollection of that meeting. And it was in the residence. The president pulled a footstool up to the coffee table and sat there very quietly, as is his nature, listening to all of the discussion up to that point, listening to Secretary Shultz, Secretary Weinberger, {then national security adviser Robert C.} McFarlane . . . . And the president listened to all of this very carefully. And at the end of the discussion, at least the first round, he sat back and he said something to the effect -- and I -- this is not a direct quote, but it was something to the effect that 'I don't feel I can leave any stone unturned in trying to get the hostages back. We clearly have a situation here where there are larger strategic interests, but it's also an opportunity to get the hostages back. And I think that we ought to at least take the next step.' "

On the contras, Poindexter said, he was "steadfast in his support of the contras . . . . I was absolutely convinced as to what the president's policy was with regard to support for the contras. I was aware that the president was aware of third country support, that the president was aware of private support."

With that in mind, he decided that "the buck stops here with me" instead of passing it on to a president he was "absolutely" certain would have authorized the diversion.