The pivot blow is an illegal strike in boxing. Last time it was used "legally" was in the 32nd round of an 1889 fight between the original Jack Dempsey and George LaBlanche. LaBlanche actually used the pivot blow twice in this fight, first time in the 26th round landing a whipping right onto the neck of Dempsey. Then in the 32nd round, Dempsey let his guard down after believing LaBlanche was ou1t of steam, when LaBlanche again did the big pivot and landing a crushing right on the bridge of Dempsey's nose sending him to the floor and winning LaBlanche the title.
Shortly after that, the New York Boxing Commission made the blow illegal. Their reasoning apparently was out of respect for Dempsey and to prevent another poem from being written -- as one appeared in this case. From what I can find, following is an excerpt of that poem on his unmarked grave, but doesn't seem overly "disrespectful" to me.
I believe the decision was based on the lines "Tis strange New York should thus forget its bravest of the brave." Google Jack Dempsey - Nonpareil - for a better history on this phenomenal fighter and you may understand New York's interest in maintaining the dignity of this man. If anyone has the full version, please post in the comments.
Far out in the wilds of Oregon,
On a lonely mountainside,
Where Columbia’s mighty waters
Roll down to the ocean side;
Where the giant fir and cedar
Are imaged in the wave,
O'grown with firs and lichens,
I found Jack Dempsey's grave.
O Fame, why sleeps thy favored son
In wilds, in woods, in weeds,
And shall he ever thus sleep on,
Interred his valiant deeds.
‘Tis strange New York should thus forget
Its bravest of the brave
And in the fields of Oregon,
Unmarked leave Dempsey’s grave
The pivot blow is a right thrown after a complete turn/pivot on heel of one foot. The boxer turns completely around often delivering it like a backhand, or even an elbow. Basically, tremendous force is generated from the acceleration of the spinning motion and there is very little control - they just let it swing.