How does the input-line test work?

[Hector Chu]

input-line.sh is:

bash ./input-line.sub
this line for input-line.sub

with input-line.sub being:

read line
echo line read by $0 was \`$line\'

The test is run by executing: bash < input-line.sh.

How does input-line.sub manage to read from the correct point in the
input file? Doesn't the parent bash read the whole lot into its
buffer, so that the child bash starts reading from the end of the
file?