A cup of espresso that has not been 'pressed' will have a slightly ashy characteristic and will be much thinner, more watery and slightly more astringent. A cup of properly 'pressed' espresso will have all the lovely texture and viscous body, similar to extra-virgin olive oil, but with the extraordinary sweet, smooth chocolaty richness that only great espresso coffee can provide.
Hydraulics and coffee? The principle of hydraulics states that pressure applied to a liquid will exert even pressure at all other points. This is critical in ensuring that espresso coffee is evenly extracted.
"If there is a gap between the top of the ground espresso coffee and the dispersion screen, the water will have to fill the entire gap before it will start exerting downward pressure on the coffee. This is bad. "