There's no need to embed the image.
The low pressure is the pressure used to fill the die with molten metal. I've been involved with some injection-molding of plastic, and the pressure kept on the die helps to keep the parts from shrinking excessively as they cool in that scenario. I'd imagine something similar with metal. The jug on the left is cast with an investment mold that is not reused, and I don't believe those are pressure-filled.
Of the two cross-sections, all else being equal, it would appear the one on the right would develop a higher compression ratio due to the smaller volume of the domed area, though the cylinder ID is smaller, too so it may be a wash.
The one on the right has open ports such that the sides of the piston form the inner walls of the transfer passages. Overall there's not so much functional difference between them in the long run, except that the piston is less-supported (though adequately so) on the right.
It looks like the intake would flow a bit more on the left than on the right, but it may be proportionally the same.
Glen
