Can I submit a proposal revision just as the auction is ending?

You can choose to submit a revision at any stage before or during the auction.

The reverse auction clock will reset to 5 minutes if there are any proposal revisions in the last 5 minutes of the auction.  There can be multiple (an indefinite) number of extensions of the auction end time.

Once the auction has ended, the client may choose to reopen a request so that you can submit additional revisions.