Inspired by this answer:
(Side note: "Reducto" would be a better counter to AK than Expelliarmus because it takes two less syllables to cast so you have a wider window, but you'd have to make sure to hit either their wand arm or their head to prevent them from finishing the curse, so...)
Many spells in the Harry Potter universe are specific in their target/range (Petrificus Totalus) or limited in scope (can't Alohomora a door that's already unlocked). Other spells, we see an effect on a whole body/object, or the immediate effect doesn't change the end result (the specific physiological mechanism of a killing curse usually isn't important).
- Reducto just one leg of someone's pants or a branch on a tree?
- Crucio someone's toe without causing them pain over their whole body?
- Imperio just someone's fist so they couldn't stop hitting themself?
- Wingardium Leviosa just the end of the bed so I could slide a rug under it?
- Accio page 5 without getting the rest of the book?
Those are just examples. We do see examples of incomplete spells when the students are in class (in Transfiguration, Ron turns Scabbers into a hairy goblet with a tail). Is it possible to intentionally use spells that seem to affect whole bodies or whole objects on just a part of them?