Answer:
The absolute phrase is "legs churning toward the goal-line".
Step-by-step explanation:
An absolute phrase is a phrase that modifies an independent clause or a noun without the use of a conjunction. The absolute phrase is not connected to the main sentence by conjunction but still modifies the noun in it.
In the given sentence "legs churning toward the goal-line, the running back plunged into the end zone for the winning touchdown", the absolute phrase is "legs churning towards the goal-line". The other part of the sentence "the running back plunged into the end zone for the winning touchdown" is an independent clause while the dependent clause "legs churning toward the goal-line" describes the player which is a noun.
Thus, the absolute phrase is "legs churning toward the goal-line".