Although I have frequently touted this simple test, I'm not sure I ever explained the reasoning behind it, or why it results as it does:
A stator is constructed with windings of insulated copper wire - at first glance it looks 'bare' but actually has a thin enamel insulation sleeve on;
The wire is wound onto the metal pillars which themselves are attached to the centre core. The coils are wound such that when one is complete, it skips two posts and then the next one, repeating every third one (the total number of coils will always be a multiple of 3); then that is repeated for every second coil then every third coil - that completes three separate 'phases'
Edit - I just noticed that stator in the image above is connected in wye not delta - I really only posted that image I found on net to illustrate the 'every third' connection - delta connection is as described below, the three do NOT come together in a single point on a delta connection.
Then the three phases are connecting in series together with the end of the third one connecting back to the start of the first one - this is called a delta connection (like triangle) and at each intersection of two phases, a wire goes out to the output connector.
So from purely static perspective, it is essentially three very long pieces of wire, connected into a triangle with a wire coming off each node of the triangle to the connector. Now even though the mean length of each of those three wire is very long, when you read the resistance of it it is extremely low because essentially you are measuring close to a dead short. Which means that when you measure from any pin to any pin, you will measure close to zero ohms.
Now - remember that the wire being used was insulated - that means that there is no connection between any of the wires and the core of the stator and if you measured the stator on the bench, you would get 'open circuit' (infinite resistance) when you measure from any of the three terminals to the centre core.
Bolt the stator into the motor and obviously the core is now electrically connected to the crankcases, which in turn are connected to the battery negative.
SO even though the stator is not directly accessible, we know that the three terminals SHOULD be electrically isolated (open) from the core and hence also the battery negative.
Making that measurement and getting that 'open' reading confirms that the insulation is intact.
Now consider a failed stator - you will almost have to take my word for this, but in pretty much 100% of failed stators, it is because the insulation has burned through and the now-exposed copper wire shorts to the core - this exacerbates the problem and it intensifies the issue. Why does it burn? Pure and simply because of heat* - ALL stators produce a LARGE amount of power and that has to be dissipated by the stator itself - oil is used to removed heat. The problem is, the oil is not equally dispersed at every coil. Again, you may have to take my word for this, but on any given bike, you will almost certainly find that if you compare failed stators from like models, it will always be in the same region where a coil fails.
So once it burns through and shorts to the core, you lose most of the current from the phase with the short. The current is now going straight to 'ground' instead of out to the Regulator, so you get reduced current and voltage output as result.
The simple test of course is to again measure between one of the output terminals to ground and a zero ohm reading confirms the issue. Why do I say ANY one of the three terminals? Because each of the three terminals is already essentially zero ohms to each other; so regardless of which actual phase has the short, all three will read that zero ohms reading to ground. So you will ALWAYS have all three terminals read good or all three will read bad (short)
Hope that explanation made things better and not worse!
* Heat - and that is why I promote the use of a Series Type Regulator - I won't repeat explanation of how that works that here, except to say that the HUGE advantage is in the amount of heat that will be dissipated is greatly reduced, lessening the possibly of the stator to fail. This is especially important in a Rocket where the entire engine has to be removed in order to replace a failed stator.
Now I'm not saying a Series Regulator is a GUARANTEE against stator failure, but it is much less likely to happen, due to the much lower operating temperature.
