The yellow circular section at the crown of the envelope is the parachute valve. By pulling on ropes in the basket the valve can be opened and some hot air released. The balloon will descend at a faster rate than if it were allowed to cool naturally.
Using the parachute valve and the burners allows very accurate vertical control of an hot air balloon.
This picture is copyright 2007 the "4-flight" balloon group. You are permitted to use this image for non-commercial use as long as the copyright to the "4-flight" balloon group is fully acknowledged. On web-sites a link to www.balloonride.org.uk must be included.