fabpot/Twig — comment on commit 1d6dfad4f55c721ec7b6e674ef225b8442bce896
File:  line 0
By Seldaek:

The original and only reason for do is to suppress the output of a
function that does output, but that you don't want to see the output of.
As it stands, doing {{ flush() }} should not be an issue, even without
do, it "outputs" null, so what?

{% do flush() %} just makes it more clear that this will not output
anything. {% flush %} doesn't say anything, because {% render %} outputs
for example.