If it had been baseball, they would have invoked the infield fly rule. But this was flying puck and the New York Islanders' Bob Nystrom circled under it for a seeming eternity, then shoved it into the open net to give sudden life to the Islanders' hopes of a Stanley Cup.

The goal that provided the Islanders with a 3-2 victory over the hated Rangers at Madison Square Garden came after 3 minutes 40 seconds of overtime and it was appropriately scored by the guy who clocked more overtime than anyone else. Nystrom had double-shifted, playing right wing on two lines, because Coach Al Arbour chose to dress six defensemen and only 11 forwards.

Yet somehow when the puck squirted across the Ranger blue line, Nystrom found the energy to pursue it, along with Ranger goalie John Davidson, who skated 40 feet out of his net in the absence of defensemen confused by a line change.

Both players hit the puck with their sticks and it sailed high into the air. Nystrom was able to corral it, about 15 feet from the goal, while Davidson was still trying to scramble to his feet.

When the red light went on, this most entertaining series was even at two apiece and heading over the Southern State Parkway to Nassau Coliseum for Saturday night's fifth game.

The decisive play began when Ranger defenseman Dave Maloney sent the puck around the boards and headed off the ice. Islander Clark Gillies grabbed it, eluded the other defender, Mike McEwen, and knocked the puck through center.

"When the puck bounded free, I just raced for it and there was no defenseman, just big John standing there with his mask on," Nystrom said. "I skated and he skated, and he was really moving. We both sort of hit it at the same time, his stick and my stick, and it popped way up.

"It was a lucky break. While it was up there, I was wishing, wishing it would come down and wanting it to hurry. I looked back to see where their men were and a man was coming off the wing, but I could see I had enough time, so I made sure of it.

"I scored last year in overtime against Toronto, but it wasn't close to what I felt tonight. Right now it really counts. This is a tough, tough series and these guys are playing unreal hockey."

Davidson said he hesitated the slightest moment before starting out of the net and it was enough to make the difference.

"My stick beat him-I was there first," Davidson said, "but he just roared right through me. I tried to deflect it by him, but his momentum carried it ahead. He just waited for it to come down, like a quarter from Heaven."

"I feel bad for John," said Dave Maloney, the Ranger captain. "John played a hell of a game to lose like that. We didn't make the change right away. I got caught in the middle and the puck ended up in the net."

If the Islanders won on a lucky break, it was one of the few they have received in this series, and it came after the Rangers had tied the score on a goal that was even luckier.

Rookie Don Maloney scored his second goal of the game and sixth of the playoffs on a shot from an almost impossible angle that struck Islander defenseman Stefan Persson and caromed into the net at 6:50 of the third period. Davidson made several remarkable saves as the Islanders dominated the rest of regulation time.

"When they scored that one, I was out there (on his other line) and it was like a kick in the solar plexus," Nystrom said. "It took the wind right out of me, I'm so glad I was able to make up for it."

The slow ice and the heat left many players close to exhaustion. Nystrom, with his double duty, was no exception.

"I'm in pretty good shape, so I guess Al thinks I can handle the extra time," Nystrom said, "but I was exhausted before the overtime ever started. After the second period, I took off my underwear and just poured water over me."

Don Maloney sent the Rangers in front on a breakaway at 1:18 of the second period. Islander rookie John Tonelli, who coughed up the puck on that one, made amends on a breakaway of his own 35 seconds later.

Billy Harris completed a superb passing sequence with Mike Kaszycki to give the Islanders a 2-1 margin at 2:56 of the third period. Don Maloney then tied it and gave Nystrom his opportunity for heroism.

Late in the first period, the 17,374 fanatics in the stands were upset when referee Bryan Lewis disallowed an apparent goal by Ranger Don Murdoch, who poked the puck from under goalie Bill Smith's glove just as Lewis blew his whistle. Greater upset would have come two hours later. CAPTION: Picture, Islander Stefan Persson gives a lift to Pierre Plante of Rangers in playoff game. UPI