Modus Operandi is Latin for Operating Mode - Wikipedia.com
This extract is referring to the sub conscious' mind's ability to process and organise actions. It is a constant process that happens even without us thinking or realising it.

The sentece actually is confusing. I think he is trying to say the subconscious is executing instructions in the brain and we are not sure how and why it does some of it. The definition is : a method of procedure; especially : a distinct pattern or method of operation that indicates or suggests the work of a single criminal in more than one crime

World English Dictionary modus operandi (ˈməʊdəs ˌɒpəˈrændiː, -ˈrændaɪ) [Click for IPA pronunciation guide]

— n , pl modi operandi procedure; method of operating

[C17: from Latin]

Word Origin & History

modus operandi "way of doing or accomplishing," 1650s, L., lit. mode of operating (see modus).

Legal Dictionary

Main Entry: mo·dus ope·ran·di Pronunciation: "mO-d&s-"ä-p&-'ran-dE, -"dI Function: noun Etymology: New Latin, manner of operating : a distinct pattern or method of operation esp. that indicates or suggests the work of a single criminal in more than one crime

Cultural Dictionary modus operandi (m.o.) [( moh -duhs op-uh- ran -dee, op-uh- ran -deye)]

The way someone does something; a characteristic method: “Her modus operandi in buying a new car always included a month of research.” This phrase, often abbreviated “m.o.,” is used by police to describe a criminal's characteristic way of committing a crime. From Latin, meaning “method of operation.”

Encyclopedia

modus operandi

in criminology, distinct pattern or manner of working that comes to be associated with a particular criminal. Criminologists have observed that, whatever his specialty-burglary, auto theft, or embezzling-the professional criminal is very likely to adhere to his particular way of operating. If, for example, a burglar begins his career by entering houses from the roof, he will, in all probability, continue this method for as long as he is able to work. Some burglars become so attached to their modus operandi that they burglarize the same places or people again and again.

The method of operating

I supose you could say we have not the slightest understanding of the method of operation.