AS DEMONSTRATED by the reams that have already been written on the subject, every little movement of the podium polka on the last night of the Democratic convention must have had a meaning all its own. For some students of such performances, obviously the firm handshake accompanied by the glassy I-don't-really-hate-you expression of both men was not adequate. Some traditionalists, including apparently more than a few in the president's trailer, were candidly admitting disappointment that the evening's program did not include the Joint Vertical Arm-Raise, known indelicately by some in the photographic community as the "armpit" shot.

But what did catch the eye last Thursday night was the adoption by President Carter of Sen. Kennedy's modified hand salute to the crowd and the meaning (in translation from its Third Worldliness) of that salute as practiced by the senator himself.

Quite frankly, the question of how to acknowledge and to respond to an enthusiastic crowd before the cheering does stop has always impressed us as one of the more awkward ceremonies for any candidate to master. Some candidates handle the situation by merely waving promiscuously, at no one in particular and everyone in general. Others, especially those who are accompanied by their mates on the platform, react by finding someone they personally know in the crowd and then pointing that person out to the mate and the mate to that person.

Former president Richard Nixon was the originator and virtually the exclusive practitioner of the individual Joint 45 degrees Arm Raise, with accompanying double-V salute. Mr. Nixon, it will be recalled, complemented this maneuver with his very own chin-in-the-chest variation. Lyndon Johnson, who was president when men still wore hats, used to wave his own Stetson to the crowds and seemed utterly natural doing it.

But Sen. Kennedy, in the campaign just concluded, developed, if he did not perfect, his own private stock wave. It reminded some older folks of the 1968 Olympic Games in Mexico City when champion American sprinters John Carlos and Tommie Smith, during the awards ceremony, bowed their heads and gave the black-gloved black power salute. The fist, in this version, is closed and facing away from the saluter. When Sen. Kennedy first offered this hand signal, at least one correspondent present Thursday night dubbed it the Reverse Rockefeller. But we're not going to dwell on that.

The point is that President Carter, perhaps as an earnest of his commitment to a new administration policy of friendship -- or something -- toward Sen. Kennedy, followed with his own version of the Kennedy fist, a modified Che Guevara, as we've came to think of it, and so, in an apparent spirit of unity, did Mrs. Carter. Right-ons all over the platform. Clenched fists everywhere you looked -- except where the clenchees would most liked to have been planting them just then.