I read the rules and they are a bit confusing but Boat US has a decent FAQ page on the topic.
Here is how I read it:
All boats less than 26 foot built after Jan 1 2020 and having a motor rated at about 3HP and greater must be equipped with a Emergency Cut Off switch/lanyard or wireless ECOS device
Any boat older that was manufactured earlier having a motor rated at 3HP or greater that was equipped with an ECOS, requires that it be used. Used means that either the lanyard is connected to your clothes, to a PFD that is worn or a wireless ECOS device is enabled.
A boat under 26 foot that has the steering helm inside an enclosed compartment ( think pilot house) is exempt.
If the boat has the ECOS, it must be used while under way with a few exceptions.
The ECOS device does not have to be used if the boat is trolling, docking or in displacement mode.
If the boat is plaining , the ECOS device must be enabled ( worn or a wireless device)
I can't wait to see how this gets enforced. I would imagine that the authorities would be hard pressed to state that you weren't using your ECOS while underway since you are not required to use it at slow speeds (displacing). What they can do however is check that you have a working ECOS, meaning it has a lanyard, the lanyard is attached to the disconnect switch, the other end has a clip or suitable device to attach it to your clothing or PFD AND that the ECOS works.
I may have missed a detail or two