You introduce the speakers, pronounce their names right, and make sure that speakers do not exceed the twenty-minute time limit. You have to pay attention because the chair may have to ask the first question to get things rolling. You then have to moderate the question and answer period. The hardest part of chairing a session is getting the speakers off. Some folks just won't stop talking.
Chairs make eye contact, make signs, pass notes, and still the speaker yammers on.
One chair armed herself with a crossbow, and THAT didn't work. The Europeans use a timer with a buzzer. When the buzzer goes off, so does the speaker, even in mid-sentence. What to expect at the conference There's more to the conference than just your paper, strange as it may seem. There are other academics there who are also intent on giving papers.