A close inspection of pictures portraying Jem fighting reveal that his stance was different from today’s boxers.
First, he fought with his palms up rather than in the modern style of “palms down”. Modern boxers use “palms down” because it is possible to hit harder this way, and with the fists encased in padded gloves there is no reason not to. However, when Jem was fighting to “London Prize Ring” rules, gloves were not allowed, and using this style makes the knuckles very vulnerable. Damaged knuckles would inevitably lead to a lost fight and in severe cases, a lost career. Also LPR rules allowed wrestling and wrestling throws. Any offensive or defensive wrestling move is more easily accomplished from a “palms up” position. Modern wrestlers use “palms up”. Although Jem was primarily a boxer and used fist blows, he always had to be ready for a wrestling attack.
Second, he stood very upright compared to today’s more usual “crouching” stances. This was because most of his opponents had a longer reach than him. He needed to maximise his reach and height to stand any chance.
Third, he kept his weight primarily on the back foot, so that he could easily step forward and produce a rapid “straight left”, fencing style. His primary movement was forward and back, rather than the side to side movement of boxers who preferred swinging blows.
Fourth, he held his arms much lower than most modern boxers. Arms are held high to ward off facial attacks. But, while this helps defensively, it reduces attacking possibilities. Jem held his hands lower to give himself better attacking possibilities. This meant he had to rely on his exceptional ring craft, quick wits, agility and stamina to avoid punishment. Although top flight modern boxers are almost certainly stronger and hit harder, very few can achieve the sort of agility to box in this way for more than short bursts. Jem could keep this up for 40 rounds. Even in his later career he could dance round much younger opponents.
This is a picture of Herbert Slade demonstrating the stance taught to him by Jem.