PrecomputedAOMask node doesn't work

I’ve tried using this node a few times, both in version 4.19 and 4.20.

I am only testing it now, using it to Lerp between two bright colours, but the Occluded mask never shows up. It seems to simply not work at all.

I’d love to use this node to do more advanced thing sin the future, such as snow and grass masking, but the node itself seems completely inert. Unless there is something else I am meant to do first.

Never Mind. Apparently there is a tick box under world settings called something along the lines of “Use Precomputed Ambient Occlusion mask”.

Ticking that fixes the problem, but it seems you have to bake out lighting to see results.