The Stanley Cup semifinals, both tied at 2-2, move into the pivotal stage Saturday night with observers wondering whether the teams involved can continue to produce the thrills of earlier games.

Thursday night, the New York Islanders' Bob Nystrom took advantage of a pop-fly deflection at 3:40 of overtime to lift his club to a 3-2 victory that evened its series with the Rangers.

Meanwhile, in Boston, the Bruins' 38-year-old Jean Ratelle was completing his first playoff hat trick at 3:46 of extra time to enable Boston to beat Montreal, 4-3, and deadlock that series.

For excitement, there is nothing to match sudden-death hockey and one can only hope National Hockey League owners soon see the light and restore overtime to the regular season, which this year was dulled by 111 ties in 680 games.

Not even the owners, however, can find a way to bring the intensity of playoff hockey to the regular season. It is impossible for a human being to sustain over 80 games the drive that players have exhibited in these semifinals.

"An overtime period is supposed to make you more tense and make you more tired." Nystrom said after his game-winner. "But I was exhausted before the overtime ever started."

He was not too weary, although double-shifting on two lines, to sprint for a loose puck at the same time that Ranger goalie John Davidson skated 40 feet out of his net to try to deflect it. The two arrived together and the puck sailed high in the air, to land behind Davidson, where Nystrom was able to push it into the empty net.

Despite his disappointment, Davidson said, "That was a great hockey game.People have been coming up to me and telling me that they haven't seen hockey like this in years."

"Everybody's got extra engery in the playoffs," said Ranger Walt Tkaczuk, whose superb penalty killing has been the key to the 0-for-17 performance by the Islanders' power play, No. I during the regular season. "Things have been going pretty good for us and that just gives us that much more of a lift."

"In playoff hockey you see so much more intensity," said the Ranger assistant coach, Mike Nykoluk. "The guys on the power play all year had it so nice and now here in the playoffs you get that extra intensity and it's tough for them to make the necessary adjustments."

(This is a tough, tough series and these guys are playing unreal hockey," Nystrom said. "You have to work, work, work to get anything. We've finally started playing pretty well but they must have played pretty well, too, since it went into overtime.

"We haven't really been getting the breaks we could, pucks have just been deflecting wide when it looks like we had Big John (Davidson) cold. But my goal was a lucky break and now maybe the breaks will start to go our way."

Hockey fans, other than those with attachments to favorite teams, tend to root for the underdog. Accordingly, admiration and support accompany the Rangers to Nassau Coliseum and the Bruins to the Montreal Forum Saturday night (WDCA-TV-20) at 8 p.m.) in their difficult challengers to teams that posted far superior records during the regular season.

However, such natural sympathy is muted by anticipation of a final series between the Canadiens and Islanders.If the semifinals can be this exciting, it figures, there are no limits to the possibilities in such a final match-up.

Montreal has won the Stanley Cup the last three seasons by overcoming the clutching, mauling tactics of Philadelphia in 1976 and Boston the last two years. Not since 1971, when Montreal overpowered Chicago in seven games, has there been a truly memorable Stanley Cup final in which swift skating, relentless but clean checking, and outstanding goaltending were all present, and intimidation absent.

The Rangers and Islanders have shown that such ingredients inject the greatest excitement into the sport.