Butter is when you put weight on your tips or tails, flexing your skis until the opposite end becomes unweighted and typically off the snow and you rotate at the same time to switch or forward, usually off a kicker onto a rail or off a jump.
Basically, its a manual (like skateboarding) and a revert simultaneously.
'landing backseat' is an expression for when you land a jump leaning back, typically out of control.
Butter is not just a revert.
'Nature does nothing uselessly.' -Aristotle