I don't see why 40% has to have such a hold on senate business. What am I missing?
Frist can't force them to talk. Under the Senate rules, a vote on the nominations can be held in one of two ways: (1) there is unanimous consent to hold the vote, or (2) 60 senators vote to limit debate and have the vote.
To beat #1, the Dems only need to have one person present to object when the Frist asks for a vote. The Dem doesn't even have to talk. Under the rules, he can object and then ask for a quorum call, which takes 15 minutes. He can repeat this process indefinitely. That's why 40 votes are needed.