I have a cube and I want to cut along an edge so that I can move a face away from the cube.

Basically I want to make a cut along the orange selected closed line and at same time keeping all the faces (I just want to be able to move away faces without deforming the cube).
Note that my initial mesh is just a cube subdivided once, but seems the functions are a bit bugged:
Here's what I tried so far:
Attemp 1
- Vertex Edit mode -> Edge Select -> Select ring to cut and press V
Attemp 2
- Vertex Edit mode -> Vertex Select -> Select vertices to make a ring and press Y

