According to Wikipedia, Zen is "... notable for its emphasis on mindful acceptance of the present moment, spontaneous action, and letting go of self-conscious and judgmental thinking." This is the essence of the relationship between Zen and motorcycles.
"...mindful acceptance of the present moment, spontaneous action..." This is its core. Whether riding down a dirt trail or five lane asphalt super highway, safety and proficient motorcycling relies on that core.
"...the present moment..." means everything. It means the weather, the road, the smells, the other riders and drivers... the everything. That is the razor's edge. That is the moment that must be accepted as a whole, without subdivision, without judgment.
According to Robert Persig, in his seminal work, Zen and the Art of Motorcycle Maintenance, "A motorcycle functions entirely in accordance with the laws of reason..." Its individual pieces work together to express several concepts into one complex machine. The levers and pistons and cables and pads all working together as one.
To be a safe motorcycle rider, and understand the true beauty and joy of piloting a five hundred pound machine down the road, one must accept these things. There is no good. There is no bad. There is only the now; the razor's edge. There is no time for anything else.
There are no brakes, no pistons, no clutch, no tires. The bike and rider must act as one. Any judgment, classification or separation of any part of this assembly reduces the effectiveness of the system, the beauty of the system. Focusing on one riding skill may adversely affect the very thing being focused on. A rider who wishes to improve their braking skill should not concentrate on that one thing to the exclusion of all others. Good braking is not simply using the right foot and right hand with the proper pressure. It is so much more. It is throttle control and balance and clutching and shifting. It is everything.
The human brain is not good at multitasking. At any moment while riding, there are a huge number of events. This is the everything that must be processed in the brain. It is more than daunting; it is overwhelming. Processing each point of data with any reasonable facsimile of efficiency simply cannot be performed.
Processing brake and clutch pressure, throttle, wind, balance, lean, surface condition, the everything can only be effectively done as a whole, not as its parts. Riding down a twisting road is not throttle and brake and clutch and lean and shift. It is simply riding. Perhaps life should be treated more like this.