On this homebuilt aircraft site I found the following:
There are several mechanisms available for improving controllability in a spin. One is as described above, where a triangular surface is
placed (in a horizontal attitude) at the root of the horizontal stab.
Practical examples of this can be seen on BD-5s and the Glastar. As
was surmised above, for local angles of attack higher than normal, the
strake creates a vortex which provides the airplane with additional
stab and elevator control, helping the airplane maintain a nose low
attitude for recovery.
A similar and more common addition is the dorsal fin - a triangular
strake placed in front of the vertical stab. This was an effective fix
(although not a cure) for the spin characteristics of the Grumman line
of airplanes, which also tended to flatten out. As the plane spins,
this strake also generates a vortex flow that is imposed onto the stab
and rudder, thus providing a force counter to the spin direction.
The last option is rarely used but it is the addition of ventral fins
at the aft end of the body. They serve two functions - they provide
more aft body area thus resulting in slightly better yaw damping and
stability and also, when in a spin they create countering lift,
similar to that of the dorsal.
None of these are fixes - they are aids for better control and for
providing the pilot with more control authority for getting out of an
Although it's not really an authoritive source, it sounds like a sound explanation to me. I can image that the vortex coming from the strake helps to maintain yaw authority with the rudder.
As such it's an alternative to the dorsal fin, which you can still see on current aircraft.
However, Here the user pontifex mentions from first hand experience that there's hardly any difference:
I have been lucky enough to fly and teach on many training aircraft and have spun them all many, many times. I have never had any problems in recovering when using the advertised technique. Some had strakes, some didn't. Never noticed any difference. That also goes for the Chipmunk of which I have flown both versions.