I don't think you are doing anything wrong. I think AnnaK's alternatives two alternatives of the sand or spider gasket are far more likely.

I normally simply backwash by going to waste, then to backwash. I wait till the sight glass is running clear for 10 or 15 seconds, back to waste, and back to filter. You turn off the filter to reduce pressure on all the components in the multivalve but also in case you release the handle in the CLOSED position with the pump running the multivalve will probably be damaged.

I NEVER have any blow-back as you describe. I don't go through AnnaK's elaborate procedure (not that it's wrong, and it works for her pool) because by doing so my pressure drops back to "normal" (for my pool, 20psi is the norm when the pump is on high).
Carl