|If every Senator were as awesome as Jeff Merkley,
this little bill would have a much better chance
of becoming a law.
The way this works is to think about three categories of outcomes for the vote to end debate. If fewer than 51 Senators want the debate to end, then debate continues under current pre-cloture rules. If more than 60 Senators want to end debate, then that's it; it's time to vote, and the bill will almost certainly pass. But in this middle ground, where a majority of Senators support a bill but not enough to break a filibuster, the rules change. The minority must engage in a "talking filibuster" and hold the floor continuously; if they fail to hold the floor, a new cloture vote occurs with a fifty vote threshold. Once that vote succeeds (which it likely will, unless Senators change their minds), the Senate moves to final passage. Amendments may be offered and debated at any point up until the second cloture vote.
Merkley's theory is that the threat of a talking filibuster will deter Senators from making frivilous filibusters. Organizing Senators to hold the floor continuously is surprisingly difficult in the modern era.
You can read the full memo from TPM's document collection. I especially like the part where Merkley points out that LBJ filed one cloture motion in his entire career, while Harry Reid has had to file 400 of them.
Pre-publishing update: Dylan Matthews put this all in flowchart form, which is always awesome.