How it works
Tattoo numbing creams are topical anaesthetics, usually lidocaine, sometimes combined with prilocaine. Applied to intact skin under an occlusive wrap for the recommended time, typically around an hour, the anaesthetic soaks into the upper skin layers and blocks the sodium channels nerves use to send pain signals. The result isn't total numbness; it takes the top off the sharpness for roughly the first one to two hours of a session.
That last part is the catch nobody tells you about. Numbing cream wears off, often mid-session, and the rebound is real: skin that's been numb wakes up to full sensation with no adrenaline ramp-up, which many people describe as worse than just starting raw. For short sittings on a sensitive spot, cream can be genuinely useful. For an all-day session it mostly moves the pain rather than removing it.
Why some artists refuse it
Plenty of good artists dislike numbing cream, and their reasons are practical, not machismo. Some products change the skin itself, leaving it puffy or spongy in a way that affects how ink sits and how the surface responds to the needle. The wear-off timing can wreck the second half of a session. And artists use your feedback as an instrument: how skin and client respond tells them things about depth and trauma that numb skin hides.