Put your toes in the toeside pocket of the tail (as apposed to heelside) and literally stand on your toe as you approach the obstacle. You are essentially attempting to pressure flip downward into the curb and this causes the board to bounce backward. It's a ricochet movement, the obstacle is the thing that flips the board. I remember when I first had someone explain this to me, it was a lightbulb moment, like it all finally made sense.
As for doing the 180 ones, you need to approach the obstacle at the slightest angle backside (as in backside grinding the bottom step), and when the board ricochets it will sort of spin frontside, and you pivot.