So the left one should remain on DC power. So what does this checklist step actually do?
Look up the description of the switch in the Vol 2, it should say;
IRS Transfer Switch
Should either IRS fail, the IRS transfer switch is used to switch all associated
systems to the functioning IRS.
So after the right IRS has failed, by placing the IRS transfer switch to "both on L" then everything that requires IRS data is fed from the operative side as opposed to those that normally receive data from the right IRS being starved of info from the failed IRS.