I practice this when crossing the street with busy traffic; cars going in a bunch of different directions.

I stare at a point in space on the ground, and allow my peripheral vision to see the cars on my left, right, and anything else to come into my vision and awareness.

Same thing with checking my blind spot before changing lanes while driving; I try not to look at a single window and instead see the whole scenery behind the car chassis, I try to see all the windows and the whole background as I sweep my vision from side-to-side.

That's my interpretation of what Musashi said.