This phrase is metaphorical; it doesn't involve real physical movement. It describes a strong emotional impact. Her victory in the competition knocked me sideways with pride.
Pro Tip 2/3
Unexpected Events
Use this phrase when something happens that is so surprising, it metaphorically 'hits' you. Winning the lottery would knock anybody sideways.
Pro Tip 3/3
Strong Reaction
It shows a significant reaction, not a mild surprise. Reserve it for big news or events. The sudden breakup of the band knocked their fans sideways.