Controlling the woofer after it has been put in motion by the signal. No extra bass caused by improper 'loose' movement after the signal.
i.e. My vintage 15" woofers, Electrovoice 15W's weigh 37 lbs each, very strong spider, rigid magnet enclosure, and a 5-1/4 lb Alnico Magnet to push/pull 'tightly' control the cone.
you could have an 18" woofer with a less strong spider and less powerful magnet, but longer throw, i.e. move more air, more bass (the iron law), but it would NOT be tight bass.