It's hard to explain to people exactly what the effect is, and frankly I don't know how or why it has that effect. I usually give some odd explanation like, "it lets some of the sound escape to create a better/more realistic sound stage." That's basically what I think it is, and how I imagine it works.
Open full sized: The Driver has more room to manipulate the air that carries the sound frequencies. Because of this, the driver is allowed more larger but faster movements to create a better articulation of the sound
Closed full sized: The Driver is enclosed or has a small area to manipulate the air around it. It has to replicate the open sound that open cans have by making the driver more in smaller but slower movements. Creates the same effect but can be changed and different ways of producing the same sound are being used.
Pretty much, the driver has more air of move and this gives the sound the driver creates more body since the sound can travel farther before it enters your ear. This happens either in the back of the driver or the front. In some cases the ear is moved away from the driver to create soundstage or the driver has more room behind it.
But open can, best if Grado for this
Cup open area
Music is diminished