Do you have a suggestion for a better wording of the callable bit or perhaps an example should be included?  I agree that callable may be a bit too open ended.

You have to do something along the lines of this in urls.py
```
from myapp.forms import MyForm

# as one of the url routes
url(r'^accounts/login/', 'django.contrib.auth.login', {'authentication_form': MyForm}, name='login'),
```

After double checking the python docs "class object" or going more specific to "form object" may be better, although technically any callable which returns a form class object would work.  A bit more detail is provided further down in the docs at lines 649 to 654 in the current master branch.