I found a paper by Microsemi as attached. This paper in my view is a very good one, which illustrates the principle of LLC operation based on physics other than mathematical equations.  

By the way, the author seems to be a very experienced senior guy who not only understands LLC truly well, but also succeeded in presenting it with a beautifully worded and intuitively understood story.

/upload/news/file/20190331/LLC_Converters_App_Note_Public (1).pdf

No wonder it's a well written article. Here is the introduction about the author: 
Sanjaya has authored seven well-known technical books, including the world's only book on Power over Ethernet. His unique perspective in power electronics is the natural result of an equally unique educational and professional voyage, one which eventually spanned several continents and work-cultures, gathering hands-on experience quietly and steadily for decades. Finally, the best of the cost-driven Far East, high-tech German engineering and American inventiveness, were distilled into a lucid, reader-friendly and rigorous tome. Sanjaya initiated the first unified approach to switching topologies, breaking them down into three fundamental cells, then re-uniting them through the far-reaching concept of current ripple ratio. He was also the first to recognize, articulate and apply fundamental power and frequency scaling principles to resonant topologies, which resulted in the world's simplest one-page design methodology for LLC converters. If these were akin to some powerful yet breathtakingly simplifying approaches in physics, it was perhaps no coincidence. Sanjaya holds advanced degrees in physics from acclaimed schools such as St. Stephen's College Delhi, IIT Bombay and Northwestern University Illinois. He ranked 17th in the prestigious National Talent Search of India. Professionally, he has worked in well-known companies such as Freescale, Siemens AG and Broadcom. He spent over five years at National Semiconductor (now Texas Instruments) in Santa Clara, California, where he created innumerable articles, widely-cited App Notes, patents, datasheets, training material and webinars. He was also National's only guest columnist in history, besides the late Bob Pease. Sanjaya lives in Fremont, California with his ever-supportive wife Disha and their two little Maltipoos.