As a result of the pipe in x | y, a subshell is created to contain the pipeline as part of the foreground process group. This continues to create subshells (via fork()) indefinitely, thus creating a fork bomb.
$ for (( i=0; i<3; i++ )); do
> echo "$BASHPID"
> done
16907
16907
16907
$ for (( i=0; i<3; i++ )); do
> echo "$BASHPID" | cat
> done
17195
17197
17199
The fork does not actually occur until the code is run, however, which is the final invocation of : in your code.
To disassemble how the fork bomb works:
:()- define a new function called:{ :|: & }- a function definition that recursively pipes the calling function into another instance of the calling function in the background:- call the fork bomb function
This tends to not be too memory intensive, but it will suck up PIDs extremely quickly.