The blade, if not pulled back, will prevent the sear from moving far enough to allow the firing pin to fall. Basically the tale on the blade blocks/limits the sear's travel if the blade isn't pulled back. You will hear a faint click when this happens, but it's noticeably quieter than when you pull the trigger & blade together and hear the firing pin release.
If it's actually letting the firing pin drop when you do this try adjusting the trigger for a heavier pull weight to see if that resolves the issue.
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