The board-directed shake-up at General Motors Corp. has sandwiched its chairman in an untenable position between two powerful forces, says a longtime observer of the GM scene.

The chairman, Robert C. Stem-pel, "is the bologna in the sandwich," said Professor Eugene Jennings, who has studied the exercise of corporate power for more than three decades. "He is isolated and neutralized."Jennings, who has often served as confidential adviser to corporate chiefs and boards, commented that "anyone who reads the corporate tea leaves" can see that power now resides just above and below Stempel.

He referred to the ascendancy of an outside direector, John G. Smale, to head the executive committee, replacing Stempel, and the promotion of John C. Smith to the presidency, replacing Lloyd Reuss, a Stempel appointee.

It is unprecedented in his experience with large companies, he said, for an executive committee to be transformed from a rather passive role of approving or disapproving management decisions to that of such active governance.

In doing so, Jennings said, the directors suggested impatience both with Stempel's management and his plan of reorganization, and he predicted that more changes will be forthcoming.

The action also produces questions about earlier board actions, he said, because it was with board approval that Stempel and Reuss assumed their jobs, and only last December that directors approved a long-range cost-cutting plan.

The question arises, says Jennings, professor emeritus of business management at Michigan State University, about whether the board feels it now has the right combination of executives in place. He said he doubts it.

Within the company, said Jennings, many of whose graduates hold important management positions, there is much speculation about the ability of people brought up in the old GM mold to break free and reorganize the company.

"You can't have the old and the new thinking trying to work together," he said. He forecast a strengthening of the positions of Smith and William Hoglund, who was promoted to chief financial officer.

"They represent the new thinking, and they'll press on to get their people into the right positions," he said. "They've got momentum."

Jennings also observed that while outside directors are exerting direct pressure on the operations of the company, the world's largest manufacturer, they themselves have been pressured by investor groups seeking change.

In recent months large institutions, once docile investors in spite of their power, have more actively asserted their management views. Other large institutional investors have expressed disapproval by avoiding GM shares.

Such attitudes, and the sense that GM management was having a tough time turning around the money-losing giant, likely have contributed to a relative stagnation in the price of GM shares.

In effect, said Jennings, author of numerous books on management and leadership, including "Routes To Executive Success," the weakness of GM's stock price suggests a lack of confidence in management's abilities.