In that video, that’s just a Monk using Hand Knife after Deprotected Zone. There aren’t any secret combinations of skills which can combine into special moves (that we know of, as far as I’m aware this isn’t a feature), but the video was referring to the fact that they’ve focused on interplay with skills a lot more.
Biggest example is Psychokino’s Psychic Pressure with either Ice Wall to shoot ice projectiles or Fire Ball to flaming projectiles. Attributes of skills allow them to combo nicely with others. Pyromancer’s Enchant Fire has a -30% fire resistance debuff attribute which you can combo with your other Pyromancer skills. Pyromancer’s Fire Wall has an attribute which raises your Fire elemental attack when on a Fire Wall tile, etc. Energy Bolt has an attribute which gains 50% more damage to enemies in the Sleep status, Elementalist can only get certain attributes if you’ve been a Pyromancer or Cryomancer before as seen in the link, etc.
There are more good combinations but this is more of the idea they were going for. Wizard’s class tree has a lot of nice interplay with their classes, I can’t say the same for other classes as I’ve not tested them yet.