Sting vs. thud
Stingy spanking implements produce a sharp sensation that is mostly felt on the skin. Thuddy implements, on the other hand, penetrate into deeper tissues and muscles. As a rule of thumb, the heavier an implement is, the thuddier it is.
Some spankees have a preference for sting while others prefer thud, and yet others like both, or implements in the middle of the spectrum that combine sting with thud, or getting a stingy implement first and a thuddy later, etc.
Thin stingy implements, such as canes, tend to produce welts and, in extreme cases, can break the skin. Thuddy implements, such as wooden paddles and heavy leather implements such as straps or belts, tend to leave bruises and, in extreme cases, can do damage to bones (such as the pelvis or coccyx) or nerves.
Imagine that a "thuddy" sensation might be that of being hit with something that has a fair amount of surface area (like a flogger), while "stingy" is created by a very narrow (or skinny) device (like a cane or singtletail).
If you hit a person with just the ends of a flogger, you will get more sting - Hitting with the entire length of a flogger will create a more thuddy sensation.
Some people add a factor of "brilliance" to the stingy sensations.
Many people "score" the stinginess or thudiness of specific toys (ie: this flogger has a 8-thud and 3-sting, or that flogger is a 6-thud and a 4-thud.)