This variable can be used to get (or to set) the angle of the instances fixture in degrees, similar to setting or getting the image_angle. However note that in the physics world rotations are calculated in the opposite way to the normal GameMaker Studio 2 game world, meaning that vector functions like point_direction should have their return values modified (simply making positive to negative should resolve this).
Real (single precision floating point value, or undefined if the instance is not physics enabled)
if phy_speed_x > 0 || phy_speed_y > 0
phy_rotation += sqrt(sqr(phy_speed_x) + sqr(phy_speed_y)) /10;
The above code checks the linear speed and if either vector is not 0, it then calculates the actual speed and uses that to set the rotation.