When consume durability of tool on hand, lower tool and lift it again.
Not happened in 1.2.5 SSP. May be happend on 1.2.5 SMP.
(1.12 as affected version. but since 1.3.1)
In normally, when player change item in hand using 1-9 key or wheel, player can see change motion. (lower current item, then lift new item.)
This motion is also happend when consume durability.
Durability is metadata value internally. It mean updating metadata.
NBT may also.