Razor cartridges have gained blades steadily over the last few decades. The addition is not cosmetic, and it changes both how the hair is cut and where it is cut.
How a cartridge actually cuts
A blade edge severs a hair by concentrating force on a very small area. The hair resists, bending forward before the edge passes through it.
In a multi-blade cartridge, the first blade catches the hair and pulls it slightly upward out of the follicle before cutting. This is described as hysteresis.
Following blades then cut the same hair while it is still extended. Each removes a shorter length, so the last cut occurs below the level of the surrounding skin.
Why that produces a closer result
A whisker cut beneath the skin surface leaves no visible stubble for a period, which is the entire commercial argument for adding blades.
Spreading the work across several edges also lowers the force any single edge must apply, which reduces the tug felt on coarse hair.
Cartridges add lubricating strips and pivoting heads, so the total experience differs from a single blade by more than the blade count alone.
Why ingrown hairs follow from the same mechanism
A hair cut below the surface must grow back through skin. If it emerges at an angle or curls back, it can re-enter the skin and become trapped.
Tightly curled hair is far more prone to this, which is why pseudofolliculitis barbae is disproportionately common among Black men and is a recognized clinical condition.
The bumps are an inflammatory response to a hair the body treats as foreign material, not an infection in the first instance.
What single-blade shaving changes
A safety razor or straight razor cuts each hair once, at or near the surface, without the lifting effect that places the cut below skin level.
The result is a slightly less close shave that regrows visible stubble sooner, in exchange for less risk of a hair becoming trapped.
Technique carries more weight with a single blade, since the pivoting head and lubricating strip that compensate for pressure are absent.
Where a professional should be involved
Persistent painful bumps, dark marks that remain after the bump resolves, or lesions that enlarge are dermatological questions rather than product questions.
Dermatologists treat this range of problems with several approaches, and choosing among them depends on the pattern, the skin tone and the hair type involved.
Changing razors may help at the margin. It is not a substitute for assessment when the problem recurs at every shave.