The Freedom to Move Rule is most commonly seen and applied on ground level. A bug cannot slide between the points of two nearby bugs, it does not fit through the space. This gap between two bugs is called a 'Gate' and one can be seen on ground level between the White Ant in your discussion and the Black Ant directly above it. This gate stops a bug from sliding into the space beyond the gate.
The same rule occasionally comes into play on the second level. It only affects Beetles, Ladybugs and bugs being moved by the Pillbug.
The Black Beetle atop one stack and the Black Mosquito atop another stack create a Gate. The Pillbug cannot pick the bug up through this gate.
Maddening? Isn't it!
I have lost a few games because of this, but I have won a few as well!