It all begins innocuously enough. Twelve men, three coaches, assorted chairs, a blackboard and orange juice, lots of orange juice.

Perhaps that's the key to the 16-minute period between the second and third quarters that the Boston Celtics have used as the first step to their dominance of the 1986 NBA playoffs, which resume Sunday in Houston.

A 34-19 outburst -- which broke open a fairly close game Thursday night and earned Boston a 117-95 victory over the Houston Rockets in Game 2 of the league finals -- was the latest in what has become a commonplace occurrence.

Just ask the Atlanta Hawks and Milwaukee Bucks. The Rockets' third-period pasting on Thursday night was tame compared to the 36-6 blitz that undermined the Hawks in the finale of the Eastern Conference semifinals. Boston also scored nine straight in one third-period stretch against the Bucks in the first game of the next series.

"I remember when I was at Virginia we used to come out strong after halftime. We just jumped on people," said Houston center/forward Ralph Sampson. "Here [with the Rockets], we're usually stronger in the fourth quarter and down the stretch."

During the opening two games of this championship series, however, the only Rockets who have been in position to make any sort of a late run have been substitutes such as Craig Ehlo, Steve Harris and Granville Waiters. The Celtics have been that dominant in the opening minutes of the second half.

"At times this team has a tendency to slack off or let down," said guard Dennis Johnson. "Now we're not doing that. If we have a team down by 10 points we want to push it to 17; if they're down by 17 we want to push it to 20 or more."

"There's a certain amount of killer instinct involved, but I think it's just a matter of experiencing strong third quarters," said assistant coach Jimmy Rodgers. "The guys know the value of coming out strong and can step up to another level in their play."

The Celtics' halftime routine isn't full of maniacal fervor; it is with calm efficiency. The first few minutes are spent looking at videotapes of the opening two quarters of play, a quickly edited package of both highlights and lowlights.

"It's really a cross-section, giving us a chance to see what has worked well and what hasn't," said Rodgers. "Then each coach has an opportunity to speak, followed by the players. After that it's having a glass of orange juice, relaxing for a second, then going back at it out on the floor. It's not like we're doing Knute Rockne speeches in there."

Of Boston's five starters, all but Johnson played under Rockets' Coach Bill Fitch when he coached the Celtics to the 1981 championship. Now, Fitch can only wonder how to stop that third-quarter blitz.

Larry Bird scored nine of his 31 points during the third quarter and was omnipresent.

"How many rebounds did he have, how many assists," asked Fitch rhetorically. "Besides, when he gets his arms flailing all around you know it's all over, anyway."

The same might be said of the Rockets' chances in the series, even if they will be home at the Summit for at least the next two games, beginning with Game 3 on Sunday.