@synapse. Note on page 6
kyun (キュン/きゅん ) or zukyun (ズキューン), is a Japanese sound effect for a heart-throbbing sensation, such as when someone is falling in love.
Similar to doki ( どき), doku (どく) that's a heartbeat.
There is also a "cute" cliche mantra moe moe kyun. 😘