This goes back a long way, so I hope that I'm recalling correctly here....
As I understand the principle, Isobarics load a second - usually indentical - active driver behind the woofer, together in a separate sub-enclosure within the speaker cabinet. The idea is to keep constant pressure on the woofer as its (short) excursion takes it out past the baffle into the room and then back behind the baffle into the speaker cabinet. The second driver should reduce the "braking" (damping) effect of compressing the air in a smallish sealed box (acoustic suspension). Bass is said to be tighter than that typically produced by ported designs.
I recall hearing an old isobaric design from Linn way back in the day and feeling that the quality of bass was pretty good, but not really impressive in either power or extension for a speaker of that size. Of course, that was many, many years ago - so I have no idea what modern Isobaric designs can achieve.
Good Luck
Marty