This variable can be used to get or to change the y component
the instances linear velocity vector and is defined in pixels per
second (for pixels per step, see phy_speed_y). Altering this for a
static instance (ie: an instance with 0 density) will turn it into
a kinematic instance.
if phy_linear_velocity_y != 0
phy_linear_velocity_y = 0;
The above code will check the y component of the linear velocity vector and if it is anything other than 0 it will set it to 0.