I'm sorry, I'm not clear what exactly you mean. Is the goal of playing dead to ambush the enemy somehow or is it to escape unharmed? Either way, it would be a fairly risky idea. Medieval soldiers (and locals near the battlefield) saw it as their right to loot the dead and wounded, and would often either kill wounded troops or drag off those too injured to move for ransom if they looked like they might be valuable. A soldier who was lying down to play dead would be putting themselves at serious risk. If the situation was so desperate as to make a soldier abandon the fight, they would most likely try to run away instead.