Noob question but I'm very intrigued how a tacho sensor works?

I now have a td42ti model which had an exposed dimple in the timing gear area as opposed to an indent on the td42, is that correct?

Apparently only the td42ti model had that, why?

I had to put a spacer on my tacho sensor for it to work and just curious if anyone knows how that actually measures and why the TI model was changed?