Here’s patents for Blendtec. I’m not quite sure if any of these describe the lid, but maybe this one, Control System For Mixers, does. I’m not an engineer so my eyes sort of glaze over reading it.
Here’s a patent from Vitamix for an interlock lid that uses photoelectric sensors.
As I mentioned above, my Cuisinart food processor has a mechanism that stops it from running if the lid is off. It is a mechanical mechanism. When you twist the lid on it pushes down a plastic rod that pushes a button in the base which lets it run.
They have used that mechanism for ages.
I mention it because I doubt a blender would use magnetic sensors or optical sensors when a cheap solution for doing this was invented long ago.
Also, FWIW, my Vitamix blender has no lockout. You can run it without the lid or even the jar. I would not recommend doing so, but you can. Of course, other blender manufacturers might do things differently.
I wonder if it is on the front control panel.
They would have had to add the feature in the last four years. I have the same Blendtec Total Blender Classic, and it likewise runs without the lid or jar. It looks identical to the linked model, so the change would have to be subtle.
I’m sure if the OP tells us the exact make and model of his blender we’ll find out what the simple explanation is.
Being an engineer, I have searched parts lists and diagrams with no result. 
If it’s a newer model, it could probably make some creative use of quantum entanglement between the lid and some sensor in the base.
Maybe the OP works for Blendtec and is trolling us for new ideas to detect the lid.
OP, come back!