Hartley is known for his short fuse and former fly-half Davies has urged Gatland's players to test his theory.
"When you play, whoever's on the field and it's the first line-out you've got to sledge [Hartley] and put pressure on him all the time," Davies said.
"If he misses the first line-out you've got to have a word, get stuck into him.
"A bit of sledging is good for banter on the field, keeps you interested.
"There has been a spat. If you get a chance to put a shot [tackle] on him you just smash him and he'll try and put a big hit in on one of the Welsh boys and it'll lift the respective sides."