Just thinking aloud here, but… if the piston is full or partially full of water and the spring compressed… in order to decompress fully and the lever go all the way up in a split second, all the water needs to leave the piston immediately. If the venting hole is, say, 5mm in diameter (I don’t know how wide it is, but maybe it’s half of that!) he water will escape slowly, and consequently the lever won’t ever give you a kick in the teeth?
I could provide you with some mathematical formulae to calculate the flow rate and rate of decompression, but it would take me months as I’d need to re-read my physics books…. 🤣
Now, imagine what would happen if the portafilter on the group (54mm diameter hole) disengages all of a sudden… 😬