Learn English – what is the idiom for doing something covertly

You might want a to use an actual, well-known economics term:

• R.O.I: "Return On Investment"

Or

• "Pays for itself"

You would then explain how the subject does this.

Or you might want

• "you can't make an omlette without breaking a few eggs"

suggesting that a price MUST be paid to get the intended benefit.

or

• "Killer app", short for "killer application"

which refers to the fact that the very first spreadsheet program for a personal computer allowed accountants|book keepers to justify the entire cost of purchasing an entire expensive PC system because it would divide their workload by a factor of 10 or more. If the computer ONLY ran a spreadsheet, it was worth the price to them. But a computer also does much, much more, like print layout for documents, art, e-Mail, etc..

You might want:

• "Get the ball rolling"

Which conjures the image of giving a boulder a tiny initial push to start it rolling down a hill, after which gravity will take over & keep it rolling.

In English, we have the infamous know-it-all: one who knows everything; hence, a person who makes pretension to great knowledge, especially one whose didactic conversational habit conspicuously reveals his belief that he has superior knowledge on many subjects; a wiseacre; a know-all; -- usually ironical. [Colloq. & pejorative] Note: the use of this term implies that the speaker disapproves of this behavior, and may think that it is unjustified.

**know all: someone who seems to know everything and annoys other people by showing how clever they are.

No one likes him because he's such a know-all.

smarty-pants is an older idiom, for one who is obnoxiously self-assertive and arrogant, as is weisenheimer.

There is the Jerkass, who might say something like, "Sometimes I park in handicapped spaces while handicapped people make handicapped faces. I'm an asshole!" (— Denis Leary, Asshole) but this is more trope or jargon.

And, as StoneyB has kindly reminded me, there is the loudmouth:

Be loquacious, often noisily or boastfully; someone who talks too much or too loudly, esp. in an offensive or stupid way

*Harvard Square: Know-it-all capital of the universe. * - Universal Hub.