Does the creature anchored make a CMB check every 5 feet he moves, or does he move his "intended" distance and then the check is made based on where the creature wants to move?
"The target's shadow becomes a flexible tether to its current square. The creature can move up to 5 feet from that square without penalty. Moving farther than 5 feet from the tether point requires the target to make a bull rush combat maneuver check against a CMB of 10 + 1/2 your caster level + your Intelligence modifier (if a witch or wizard) or Charisma modifier (if a bard or sorcerer). The target takes a –1 penalty for every 5 feet of distance between it and its tethered square. Failing this check means the target's move is wasted and it cannot move farther away. If it fails this check by 10 or more, it is pulled 5 feet toward the tether square and is knocked prone. If it beats the check by 10 or more, the spell ends. This spell does not work on creatures that do not cast shadows or reflections. If the target uses a teleportation effect or leaves the current plane, the spell ends."
If the target wants to move more than 5 feet from the anchor square they need to bull rush. The result of the bull rush determines how far they move. Next turn they get to try it again likely with a penalty based on their new distance from the anchor square. Intended movement never enters into it.
I am wondering how involuntary movement interacts with this spell. Say your anchor square is on a tiny ledge on the side of a mountain. You then jump. As you are a 1st level gnome sorcerer who dumped STR you fail the bull rush and invent bungee jumping.