If you have a suspect in front of you, can you be watching his hands and feet while at the same time still keeping your head on a swivel and watching the whole area around you? No.
This is a problem in defense training. We are taught to watch things like the hands and feet, especially if we think a person is a threat. We are taught to keep our head on a swivel to know what is going on around us all the time. This leaves a conflict of tasks. If you look around, with a brief glance, you might spot an obvious threat around you, but you may also miss the known potential threat's action to harm you. If you don't look around, you may miss things around you, but are on track to either keep your suspect from taking advantage of your head-on-swivel security distraction or reacting faster by being able to see the start of his action. You have to prioritize your threat environment and you can't do everything all the time. If you have an uncontrolled potential threat in front of you, you darned sure can't be stopped and looking inside every vehicle, under them, behind behinds, trees, etc. and looking for other potential threats while still keeping a watch on the known potential threat...unless you have a team. However, the team needs to act as a time and not as a series of individuals. This group of officers acted as a series of individuals and not as a team.
Agree no matter how much we train. We can not train for every possible scenario, and even if facing multiple threats we are vulnerable even with keeping adequate situation awareness.
The best way to stay alive is to avoid conflict. We train in the event that escape is no longer an option or our personal life is in danger at home.