With surprise and uncertainty. I'll refer you to the fourth chapter of Slavinsky's The Japanese-Soviet Neutrality Pact: A Diplomatic History 1941-1945 and Lensen's The Strange Neutrality: Soviet-Japanese Relations during the Second World War, 1941-1945.

It's important to note that there were multiple reasons for the Japanese to sign a neutrality pact with the Soviet Union in early 1941. The Soviet military victory at Nomonhan was one, the German signature of a non-aggression pact was another, but an oft-forgotten reason was that the Soviet Union was supplying Chiang Kai-shek's Nationalist Chinese forces. The neutrality agreement with the Soviet Union meant this covert supply ended.

When Germany invaded the Soviet Union on June 22, 1941, Japan was not informed ahead of time. This precluded any preparation Japan might have made for war with the Soviet Union. On July 2, 1941, Japan's highest officials held a meeting with the Emperor to decide their future strategy. In a brief document (link is translated) those in the meeting decided that in order to prosecute the war in China, an advance southward was needed, not one northward. "Japan will risk a war with Britain and America for the sake of achieving the present objective," the meeting concluded.

In regard to the German invasion:

"As regards the German-Soviet War, Japan's actions will be based on the spirit of the Tripartite Axis. Buth without involving herself therein for the time being, she will secretly complete military preparations, and deal with the situation independently. Meanwhile, diplomatic negotiations will be carried on with utmost care. In case the German-Soviet War develops to the advantage of Japan, we will use armed force and solve the problem of the north, ensuring thereby the security of the northern frontier.

In executing the program mentioned above and in deciding upon various measures, especially upon the use of armed force, care will be taken not to cause great handicap in the maintenance of the fundamental position for war with Britain and America."

As a result of the July 2 meeting, the Japanese General Staff and War Ministry drafted plan Kantokuen, the invasion of the Soviet Union. From July through August 1941, Japan covertly mobilized the Kwantung Army, which doubled in size. On July 5, Japan created the 5th Fleet for operations against the Soviet Union.

The general staff ordered an increase in provocations against the Soviet border, intended to test its strength.

Simultaneous with this, Japan increased diplomatic pressure on the Soviet Union. When the freighter Kehi Maru was sunk by a rogue Soviet mine in fall 1941, Japanese diplomats took a hard line, seeking a formal investigation, compensation and more in recognition of the 160-odd people who died when the freighter sank.

Nevertheless, events overtook the Japanese preparations. On July 26, the United States froze Japanese assets in America (incidentally affecting Japanese-Soviet trade, conducted to a large degree through American banks), and on Aug. 1 it began an oil embargo against Japan.

This embargo effectively started the clock toward war. Japan imported 80 percent of its oil and refined petroleum from the United States, and while its civilian economy could muddle onward without petroleum, Japan's military ─ and the prosecution of the war in China ─ depended upon it. A southward movement to seize the oil fields of the Dutch East Indies was needed promptly, and the Imperial Japanese Navy believed that the United States and British forces would intervene if Japan attacked the DEI on its own. Thus, attention moved south, and the Soviet Union became a secondary consideration.