Like most things speaker design related, it's well understood ancient history and easily simmed in all the versions of LspCad AFAIK and of course AkAbak or similar programs. I forget who did the initial work, but nowadays it's usually referred to as a double bass reflex [DBR], though there's three different variants of which the M19 is the simplest variant used to improve driver damping in the 1/2 octave above a low tuning where the driver is most vulnerable to over excursion.

In short, the horn cavity plus spacer acts as a band stop filter [Helmholtz resonator], so nothing 'leaky', just good old applied engineering, once again proving the old adage that those who don't learn from history are doomed to repeat it : http://users.cms.caltech.edu/~ps/All.pdf

Regardless, a well done MLTL will provide better damping overall over a wider BW, so DBR loading is best left to improving BR power handling. Note too that it creates a notch in the mid-bass as the trade-off same as what occurs with a MLTL if the vent is located close to the driver to achieve this type of notch filter, so no 'free lunch'.

GM