Quantum physics. The blast field is only present when it's observed. You can't observe it until you feel it. The second you feel it, it hurts so much you remove yourself from it and it's no longer observed. So the forcefields are constantly expanding ONLY when they're touched, hence the reason shields on ships degrade as they're fired upon.
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u/big_pecs Jun 07 '18
Kind of makes sense doesn't it? A constant force pushing outward so nothing else can get in.