[mSp]

pawn Code:

forward CheckHealth();
public CheckHealth()
{
for(new i = 0; i < MAX_PLAYERS; i++)
  {
    new Float:health;
    GetPlayerHealth(i,health);
    if(health <= 20)
     {
        ApplyAnimation(i, "ped", "KO_spin_L", 4.1, 1, 1, 1, 1, 1, 1);
        TogglePlayerControllable(i,0);
     }
   }
   return 1;
}

Not tested though..anyways would be better to use OnPlayerUpdate for health checking since it checks health very frequently