Meaning of the figure of speech

[Ruma]

When I won the trophy you could have knocked me over with a feather

[MikeNewYork]

When I won the trophy you could have knocked me over with a feather

This means that the recipent was so surprised, that he/she was weak in the knees and even a slight push would have knocked him/her down.

It is meant to be taken figuratively, not literally.

[Red5]

When I won the trophy you could have knocked me over with a feather

Also, this is probably related to expressions such as:

Well, Blow me down! - Similar to "knock me over with a feather"

Blown away - to be very impressed or surprised

to bowl someone over - to make a very good impression on someone

I hope this helps! Feel free to ask if you need more help.