That's what he's talking about, Jobo yes.
It's used in Muay Thai all the time. Thai leg kick inbound at my lead (here my left) leg, and in my stance I lift the left knee high, allowing the leg to go relaxed, which absorbs the energy of the kick... and if I want to as the kick is being absorbed I can slightly extend the leg and open up the guy... and as Bill said, Hit them. This was one of the neat-O counters I did have in my toolkit out of TKD. Opponent leg kicks me, I Thai block it, send him slightly to his right/my left... left foot drops and the TKD twitch switch-kick fires off and I thai kick them back... in the hip, lower belly or lower ribs, depending on what was open at that instant. I hardly ever had an opening for the switch-kick to the head, most of the Thai fighting guys I was working with had very good head coverage with their hands/arms. But... it left their midsections open. Normal Muay Thai stuff.
And yes, you an move back and out of range... but then you merely start the engagement over again and have gained no advantage. You always want to try to create advantage tactically, not merely extend the engagement.