In 1971 the 'Nixon Shock' removed the convertibility of the US dollar to gold, allowing for the US to implement a fractional reserve lending system where for every 1 dollar a bank held they could lend out 10x that amount. The theory being not everyone would withdraw their money at once.
A US shutdown initially won't have much effect on their economy but eventually if an agreement is not reached the US will default. When it does, it will not be able to borrow as much money from the privately owned Federal Reserve and will be forced to cut spending, which in turn causes a downturn in the countries economic growth. The returns for investment into the US stocks, bonds and infrastructure are reduced and investors look else where to grow their wealth.
This wealth exodus has been described as a 'digital run on the banks' and would bring the US economy to a grinding halt as 9/10ths of the wealth can not be honoured. No longer being able to lend money to other countries the US further losses revenue and those countries are forced in to the same position where their governments can no longer operate due to lack of funding.
That is the 20.6 Trillion dollar debt bubble