As far as i know knockdown shield have some significant delay between visual and actual protection. It affects not only moment of switching it on, but also it's angle change - to successfully protect yourself from being finished off you have to try to predict the point of enemy next shot and turn towards it significantly before the shot happens.
I'd say devs should've been asked about this long ago, if it's balance feature, server tickrate issue or just an animation-caused bug.
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