Get a diamond sword and a bow. Put the diamond sword in your first hotbar slot and your bow in your second hotbar slot. Select the sword, summon a mob to fight and attack the mob. By attacking only when your attack strength bar is full, you should kill the mob in 2-3 shots. Now, select the bow. Open your inventory, hover over the sword and press '2' to swap the sword and bow (So you just swapped your selected item from within your inventory). Summon another enemy and attack it with the sword again (WITHOUT DESELECTING THE SWORD SINCE YOU SWAPPED ITEMS), and you should notice that it takes many more hits to kill the same enemy and the attack strength bar indicates a higher attack speed, despite the fact that you're holding the same item. In fact, it seems that when swapping items in this way the game continues to apply the attributes of the item swapped out and doesn't apply those of the item swapped in.

If this in confusing, say so and I'll make a video.