You will need a switch as without one you cannot turn the head tracking on. You could turn the screen into a switch to achieve this, but you would need to be able to use your hands to operate this.
From what I have tried using this feature has some serious limitations and I have not found a very practicable way that someone who has no useful upper limb function could use it.
We usually use head tracking with dwell click, but I could not find out how you can enable the dwell function, which is in the Touch section. I do not understand why the head tracking is in the switch control section as these are both separate and usually independent means of accessibility.
I have provided feedback to Apple on this issue, but it has not as yet been addressed. It is a shame as the head tracking would be a great feature, but they need to understand how people would actually use it so they can build in the required features to make it useable.