you need air in a shock for it work correctly..but the air must be kept seperate(in the shock cap above the diaphragm).
air can compress,a liquid cannot so if you filled totally with oil the piston rod would lock solid as you tried to move it into the shock body,hence why you need the air.
pack is when the oil is asked to travel thru the hole in the piston faster than it physically can..so you can pump the shock nice and slow and its fine,try it fast and it feels like the shock has locked up momentarily(in effect you trying to squash the oil faster than it can escape..and since oil cannot compress you get a hydrolock effect)
if you take a shock and remove the spring,stand it on the desk holding it by the plastic rod end and push the shock up and down it will move freely,now give the top of it a sharp hit with the flat of your hand and you will feel the pack effect if the piston hole is small enough.
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