This variable can be used to get (or to set) the previous y position of the instance within the game room physics world. This is the position of the instance within the physics world in the previous step to the current one
Real (single precision floating point value, or undefined if the instance is not physics enabled)
xx = phy_position_xprevious;
yy = phy_position_yprevious;
The above code stores the previous x and y position for the physics enabled instance in two variables.