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This will expand once before find runs, not per file found.
Problematic code
find . -name '*.tar' -exec tar xf {} -C "$(dirname {})" \;
Correct code
find . -name '*.tar' -exec sh -c 'tar xf "$1" -C "$(dirname "$1")"' _ {} \;
Rationale
Bash evaluates any command substitutions before the command they feature in is executed. In this case, the command is find. This means that $(dirname {}) will run before find runs, and not while find runs.
To run shell code for each file, we can write a tiny script and inline it with sh -c. We add _ as a dummy argument that becomes $0, and a filename argument that becomes $1 in the inlined script:
$ sh -c 'echo "$1 is in $(dirname "$1")"' _ "mydir/myfile"
mydir/myfile is in mydir
This command can be executed by find -exec, with {} as the filename argument. It executes shell which interprets the inlined script once for each file. Note that the inlined script is single quoted, again to ensure that the expansion does not happen prematurely .
Exceptions
If you don't care (or if you prefer) that it's only expanded once, like when dynamically selecting the executable to be used by all invocations, you can ignore this message.