I'll try and explain - but I'm not as good as Adam D :roll:
The plunger was fitted onto the side lever press.
The glass was press moulded using a side lever press (usually hand operated). The (jointed) mould would be positioned so that a gob of molten glass could be dropped into it - then the plunger would be lowered into the mould, forcing the molten glass into the space between mould and plunger. A uniform thickness would be attained through the skill of the glassmakers (ie knowing exactly the right amount of glass to cut off and drop into the mould, plus the skill of the presser = experience).
Two further pieces of metal work would also usually be used - a base plate and a top ring. These two effectively stopped the glass coming out at the top and bottom of the mould/plunger.
The interior pattern is that which was cut onto the plunger. The exterior pattern is that which was cut onto the mould.
The plunger would be withdrawn upwards (the lever of the press would be operated to lower and raise the plunger) and the mould would then be opened up, and the glass removed. Mould seams can usually be seen on press moulded glass - indicating the joints of the mould.
Sorry if this isn't clear, but I'm doing my best!
I have operated a side lever press, and it's quite a challenge I can tell you.