This variable can be used to get or to change the y component the instances linear speed vector and is defined in pixels per step (for pixels per second, see phy_linear_velocity_y). Altering this for a static instance (ie: an instance with 0 density) will turn it into a kinematic instance
Real (single precision floating point value, or undefined if the instance is not physics enabled)
if phy_speed_y != 0
phy_speed_y = 0;
The above code will check the y component of the linear speed vector and if it is anything other than 0 it will set it to 0.