The phrase "as he underlined" is correct and usable in written English.
It can be used to refer to something that someone has emphasized or highlighted in a text or conversation.
Example: "The main point of the discussion was clear, as he underlined the importance of teamwork."
Alternatives: "as he emphasized" or "as he highlighted".
He echoed Shapiro, or Shapiro echoed him, as he underlined the delicate balance that lay before his young country: negotiating rapid modernization combined with an ardent desire to remain committed to one's culture and heritage.
He came up with several models for social responsibility as he underlined that CSR involved a degree of voluntarism, as opposed to coercion.
The unbeaten Dawn Approach made it look easy as he underlined his 2013 Classic credentials with victory in the Dewhurst Stakes at Newmarket.
In the men's competition, Delhi silver medallist Kirkbride came within a whisker of a new Scottish record of 191kg for the clean and jerk as he underlined his medal potential for the event to be held in Glasgow's Clyde Auditorium.
Suck on that, Cameron (one can imagine Clegg saying as he underlines the passage crossly).
And, as he underlines: "These are voters who do not forget … and if Labour is smart it will press home this point repeatedly.
And Sheepshanks' passion for the project pours out as he underlines the standards he expects the facility to set.
He underlined the impact of dietary energy intake as well as sedentary lifestyle and drugs, (e.g. plumbagin) as activators of adaptive stress response pathways, on cognitive aging.
As Reuters reported at the time: "He underlined the last eight words to emphasize his reference to the NFL football end-zone celebration".
He underlined what the company sees as the most pertinent and important operational issue in cloud today: the need for cloud mobility.
