http://tpmdc.talkingpointsmemo.com/2011 ... budget.php
Quick summary:
Instead of voting "no" against a radical Republican budget bill, the Democrats withheld their votes -- that is, they voted "present" to indicate that there was a quorum, but did not say "yes" or "no" to the bill itself. This meant that the bill would pass or fail purely on Republican votes -- and the Republicans started changing their "yes" votes to "no" and defeated the bill without any negative votes from the Democrats.
Obviously this means that many representatives were placing their votes purely for political reasons, but when faced with the possibility of the bill actually PASSING, they decided not to pass something that would be that unpopular and dangerous.