Here’s a little bit from the man himself on how he did it:
In the first place, immediately I start to press the weight away from the shoulder I become perfectly oblivious to everything except the weight that I am lifting.
The spectators are obliterated from my mind by the effort of intense concentration which is necessary to enable me to press the weight. I immediately engage myself in a terrific struggle in which the weight and I are competitor, and only one can win, either the weight must be lifted or else I fail.
This concentration is, of course, one of the secrets of success in lifting, as I have explained in another part of my book. It enables me to bring forward the last ounce of pushing power, and for the time being to exert strength beyond that normally possessed.
As the weight steadily rises aloft perhaps half way it wavers, the balance alters, and I have immediately, yet very carefully and quietly, to adjust my position to the altered balance of the bell.
Then I must proceed with the press, my body gradually falling lower toward the left knee, my eyes fixed all the time upon the ponderous weight balanced over my head, ready to fall at a moment’s notice should I weaken or place myself in a false position, and should at this moment anyone shout out, it might startle me, make me waver, and cause the weight to fall.
Therefore, if I am attempting a world’s record in this position, I generally ask for complete silence until I have either failed or succeeded, and I might mention here that to think of failure is to fail, and I always tell myself all the time that I am certain to succeed even though I am attempting a weight more than I have hitherto lifted.
Eventually, my arm is straight, and before coming to an upright position I engage in another tussle with the enormous barbell, in which I have to exert all my will power to hold together the flagging powers of tired muscles, which have been strained by the tremendous pressure which 350 lbs. brings on to them in the effort of pressing aloft.
By supreme effort of the will I fix the bell in a good position and then stand upright. Often the bar will roll on to the fingers instead of being directly over the wrist, in which case severe pain is inflicted and I have to persevere with the lift under doubly hard conditions, or drop the weight and try again.”
For more information about Arthur Saxon and his training methods, pick up copies of his two great training books: >The Development of Physical Power (1906) and The Textbook of Weight-Lifting (1910)