Since many of the exercises cycle between the states of back open / front closed and vice versa...
Hey Mert, yes, good point.
At 1:18 in this video, you can see Akuzawa open and then close very quickly.
Watch his right hand come out - as he opens, and then the hand returns as he closes - just the same as the rising force at the bottom of the Slinky, and the "return stroke" in the lightening strike.
Open = chin up, chest forward, lower back arched, butt stuck out. = Convex arch
Closed = chin down, chest sunk, lower back straight, butt tucked in. = Concave arch