Shanks is a sneaky little cheat. He is the only person aside from Joyboy who is making use of the ability to pour haki into objects and access it for later use. He is stacking every fight against his opponent because he just shows up with way more haki.
Additionally CoC is able to supress fruit abilities. Joyboy's burst forcefully disabled the summoning ritual of the Gorosei and disabled their monstrous forms. Both of those are Imu's abilities and they felt it when it happened from the other side of the planet.
Shanks supressed Aramaki's tree form and was able to get through Bartolomeo's barriers, which nobody has been able to so far.
For that effect you either need a large amount or a special type of it.
Shanks is using his sword as the object he imbues with his haki and also as a focus to channel it in more precise ways.
At the highest level fights are slugfests and the outcome determined by endurance. He circumvents all of this.
Luffy is learning this ability right now as the technique to defeat the holy knights is the basis of the trick. Luffy's focus will be the straw hat and it will turn out that the predecessors already filled it up with haki. The line of succession was only broken once and that straw hat lies in a refrigerator in mary geoise
>>280505984
CoC nullifies all fruit powers

so we know that you can imbune objects with haki. We also know that a person does not have infinite amounts of haki any given day and can exhaust it. What if imbuning can act as battery for haki, to draw on when you run out.