When you have ABS, you have sensors on each wheels that calculates the rotation so when braking, the system will know if a wheel turns slower than the others, so brake pressure will be released to prevent an incident.
Those sensors aren't used when the brakes are not depressed, so BMW used them to calculate the rotation while driving.
If a wheels turns more than another, the tire's marked as running low on air, thus the TPMS light's turned on...
We use the same here in Canada.
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