HOW WOULD you feel about picking up your cello, planting it about 8 feet from Mstislav Rostropovich, and playing under his eagle eye the opening movement of the Shostakovich Cello Sonata, a work that he can play either the cello or the piano part of with equal authority?

That is exactly what a young cellist did last Friday night at Catholic University to open a two-hour master class. As the first student began to play, Rostropovich became immovable, watching every move the cellist made, taking in at the same time every detail. Any time the cellist looked up from his music, his eyes met directly those of the master.

Not until the first movement was finished did Rostropovich say a word. At that point, his first move was to turn to the Steinway behind him and, playing a phrase from the sonata, ask, "Play for me once more this reprise." From then on, with no music in front of him, he analyzed every detail for both players: "No, no (to the pianist) that is F sharp, not F natural." Switching from minute details, he began talking about important matters for every performer:

"Each small thing, in coming to public, is very noticeable. Talent shows from the first moment of coming on stage." Gesturing toward the player's music stand, he went on: "If you fool with your music, you come to stage the same like coming to stop. Music," and here his voice took on a special, almost missionary tone, "music always begins with silence."

"Player tunes up - not make dirty for public." Here Rostropovich imitated strings being tuned up with a sound words cannot approximate. "Very pianissimo is enough," and he barely tapped the strings to check the intonation.

"Always chamber music played with music," he continued. "Partner has brought music, so you not want to show that partner has not been working enough, you bring music too. But now, with music stand between you and public, maybe 2,000-3,000 people, you have eyes on music. Pretty soon public has eyes on music stand too. You lost much of their attention. Without a stand, you free very much your emotions."

Returning directly to the young cellist playing for him. Rostropovich said, "I do not have feeling that you have much feeling on stage that you must play music. In the middle of the second theme, you play beautifully - then turn page - play again . . ." All this time Rostropovich is making a pantomime of a cellist hurriedly turning pages and getting back to his bow and strings.

Suddenly the experienced public performer remembered something else: "Very important! Not to put mute in pocket! Forget which pocket and look for mute in wrong pocket, or maybe pull out all stuffing out of pocket."

Naturally all of this was accompanied by a mime routine that was at times hilarious. "No, put mute on chair."

Referring to the musical problems in the Shostakovich, Rostropovich turned to his Steinway and began to play as he talked. "Give some line of musical idea, long line. Play for me now." As the cellist and pianist began again, Rostropovich conducted. "You make separate phrase. But music like apple pie - I prefer not all small pieces. Make long line. Play second theme."

To the pianist: "All your emotion give for this theme!" And he showed exactly what he meant on the piano, like the master pianist that he is. Then, taking the cellist's instrument in hand, he produced a single F sharp, colored by a vibrato of consummate beauty.

Another idea hit him, but for this he turned to his translator in order to say precisely what he meant. She translated his Russian phrase. "I don't yet feel the birth of a common idea between you. How long have you been playing together?" The answer, "Three days," made apparent the reason for the lack of common ideas.

A violinist came next, with the opening movement of the Brahms D Minor Violin Sonata, which, it soon became apparent, Rostropovich knew as intimately as the Shostakovich. "What is the feeling in the music?" was his first question. "Always what is the feeling in music? You," to the violinist, "are a bit too near one expression and to another expression. Must be more different. You must think in this phrase like an angel," and he played over the opening phrase.

When the first fortissimo failed to bring out a signal attack, the teacher stopped the duo and asked the violinist, "Are you married?" There was a negative shake of the head.

So he instructed: "When wife tell you something, you must not be so calm." When the laughter subsided, he gave some of the finest advice any young artist could ever hear. "Before you sleep, you must think - not play - what does music mean? Then give for us composer. Make us see composer." For a windup to the violinist, he said, 'Technique is very good. But technique come second. Think, 'What kind of sound I must produce?' Scales four hours a day, I think that is very bad. Think 'What is music?"

A string trio came next, playing the opening movement of the C Minor Trio of Beethoven's Opus 9. Again it was music Rostropovich knew inside and out. "I have played many times this trio." At the end of the session, he added. "I have many years ago recorded this trio. I am very touched that you play it. It is not so often played by chamber musicians."

To the three players, "Must all be the same: spiccato, bowing, unified all technical things. Viola must play with same spiccato as violin. And also there is polyphony not only for three instruments, but polyphony of three emotions."

Finally out came the bravest of the brave, a cellist to play the prelude from the D Minor Suite for solo cello, one of those test pieces that challenge even the greatest players. For him Rostropovich again had both general and specific assistance: "Remember the weight from shoulders - you play with a small part of the bow. You must play with whole bow. If you don't need all the bow, you should cut in half or in little pieces."

"You give too much too soon. And too much vibrato for Bach. You have lot of time for a buildup. You know legato for right hand with bow. But left hand also must have legato. Your left hand must be more like spaghetti boiled, not so dry spaghetti as you make now." This comment was accompanied by some rather cryptic chopping noises.

"Left hand jumping should be not so hard, as if trying to catch fly." The humor of these admonitions was boosted by practical assists, such as the master taking the student's right arm and showing him precisely what he meant. There followed a fluent and pointed discussion of the "fantastic" importance of the harmontic foundation upon which the solo suites of Bach rest, a reminder that music is "art in time." And, at the end, on the piano, a magnificent demonstration of Bach's use of pedal point to build harmonic tension prior to a final resolution.

By the time it was over, the term "master class" had acquired a new dimension.