I summarized it when I reminded Junio on this topic last time and it is in the same discussion thread: http://thread.gmane.org/gmane.comp.version-control.git/135325/focus=136216
Here is an extended version.
'git branch --will-track origin/topic topic origin/master' was proposed as a way to fork a topic branch at origin's master. Later 'git pull' will merge topic from origin to topic.
This is bad in two ways. It will force users to decide when the branch is created. It will not allow users to configure branch.topic.rebase variable.
'git push --track' was suggested as a way to let users delay that decision.
'git branch --configure' to update the same information for an existing branch was suggested as an alternative UI. An added benefit is that this approach will allow the same option to be used when creating a branch.
'git pull --remember' that remembers the options used from the command line was suggested as a solution in addition to 'git branch --reconfigure'. Users can postpone the decision even more than 'git push --track', and it naturally supports setting branch.topic.rebase with 'git pull --rebase --remember'. It also has two additional benefits. 'push --track' configures what happens when you 'pull' (counter-intuitive), but 'pull --remember' makes 'pull' to change the setting used by 'pull' (much more natural). Also it does not add the confusing word 'track' to the interface (for a more detailed discussion on 'track', see http://article.gmane.org/gmane.comp.version-control.git/136785).