I have never heard of planting a spear on the ground and incapitating an incoming horse, i would love to see a reliable source showing that.
And don't get mixed up with spears that were planted deeply in the ground.
The only way i suppose a grounding could be done is sticking the backtip between the foot and ground.
I couldn't find an exact photo but something samiliar (read on)
http://www.tigerandcrane.com.au/images/Asifu-spear.jpg
Now imagine the back tip of the spear is stuck between the ground and the leg, the spearman leaning forward instead of backward like the pic.
The spear can have a last second fine tuning like a couched lance, and the spearman can rotate around the backward leg by taking big steps (you can test it with a broom, just remember a spear is twice as long and much much heavier)
I don't think just sticking the spear on the ground would work, and i assume the above would result in a dead horse and a crushed spearman.
Don't get me wrong, i would love to see a way for a spearmen to couch a spear against incoming horses, or at least gain speed advantage (of the incoming horse) if he succeed in hitting them