Any right click item in the offhand (maybe like a fishing rod, flint and steel or a block) will have their animation for placing, using etc. still in the mainhand, making it look like youre swinging a invisible fishing rod or placing a block magical from your empty hand.

To recreate:
1. Put any right click usable item (blocks etc.) in your offhand
2. Go into F5 Mode
3. Place/use the item in your offhand and you will see that the animation for placing/using still just effects the mainhand