Tango K said:
I think it means “rhythm” or, more properly, “pace”. In my native language, not Korean

)) but east asian, those 3 words are the same. You can also guess through the videos. He says it whenever the learners are moving/hitting a little too late or too early.
Tango has said it about as right as anyone.
The Koreans love to use the word "Bak-Ja" a lot when describing how to step to the ball and impact at the right time.
The word Bak-Ja can literally mean the beat/rhythm, but in TT, it is how you have timed your step to the ball, your timing of planting foot, and your movement to = through impact.
An example would be that a player just doesn't step to the ball and impact it the very moment the foot hits the ground. That is too early (unless doing it dynamically in air before planting due to lack of time on the move to FH).
The recommended Bak-Ja for say a flip, would be to step in with foot under table, wait a little for ball to come deeper into impact zone, continue and channel that energy, then impact it.
The Bak-Ja in TT is meaning the when (moment + timing) and how long (duration) of the individual movements and impact.
In the English speaking TT world, I rarely here it expressed like this. Often, it is about "time it better" or something like that.