When you put a Fishing Rod in your left Hand, and go in 3rd Person and cast it, your Right arm swings as if it were in your right Arm, aswell as when you have a Block in your Left hand, and place it, your Right arm swings..

I thought my Left arm would swing when I casted my Rod/Placed a Block

when I casted the Rod/Placed a Block, my Right arm swung instead.

#1 Put a Rod/Block in your Left hand Slot
#2 Go in 3rd Person from the front view
#3 Right Click with the Rod/Block in your Left hand, and your right arm will Swing, instead of Left arm