In terms of version control, every change is separate branch, which is isolated from other branches and most importantly, from the main branch until it is reviewed for correctness and integrated. You can't propagate unverified code.
To start an implementation of tasks or to bind already implemented tasks to a source code you have to create a change. Privileged users can find the menu item New under the Change menu or on the front page in the My changes section a link New change.
You have to:
You can also:
Here is an example of this form: