Break is the small fold of fabric that forms where a trouser leg lands on the shoe. It is one of the few fit details decided at the very last stage of tailoring, and it is largely a matter of geometry.
The fold is caused by interference, not by cut
A trouser leg hangs straight until something stops it. The shoe stops it, and the excess length above that point has to go somewhere.
It goes into a horizontal crease just above the instep. The more excess length there is, the deeper and more numerous those creases become.
So break is not a style applied to a trouser. It is the visible consequence of choosing a hem length relative to the height of the shoe underneath.
Hem width decides how the excess behaves
A wide hem has more circumference to distribute the fold across, so a given amount of extra length produces a soft, single crease.
A narrow hem has nowhere to put the same excess, so it stacks into tight ripples that read as bunching rather than as a clean break.
This is why the same inseam measurement looks tidy on a wide-legged trouser and untidy on a tapered one, and why tailors ask about leg width before setting length.
Shoe height moves the landing point
A boot raises the point at which the trouser is interrupted, effectively shortening the leg. A low sneaker lowers it, effectively lengthening the same trouser.
A difference of an inch in shoe height changes the appearance of a hem more than most alterations would, which is why fittings are done in the shoes the trousers will be worn with.
Men who wear one trouser across boots and thin-soled shoes are accepting that the break will be right for one of them and approximate for the other.
Cuffs add weight and change the drape
A turned-up cuff adds fabric weight at the hem, and that weight pulls the leg down and holds the line straighter.
The added weight tends to produce a cleaner, more defined break, which is part of why cuffs and fuller cuts have historically appeared together.
On a very slim or very light trouser the same cuff can look heavy and disproportionate, since the weight overwhelms the fabric it is attached to.
The preferred amount has moved with the silhouette
Fuller cuts have generally been worn with more break, because the extra width absorbs it. Slimmer cuts moved toward no break at all, showing the full shoe.
Recent years have brought wider legs back, and the deeper break has returned with them, which suggests the two move together rather than independently.
Treating break as a fixed rule tends to produce trousers that look wrong on the cut they are attached to. It is a consequence of the silhouette, and it follows it.