Using Chewing Tobacco will refill your Dead Eye meter.
After selecting the chew it will show the player putting the Tobacco in Marston's mouth.
Occasionally, if the player would use chewing tobacco when Marston's Dead Eye meter is full, Marston would put the tobacco in his mouth, then proceed to spit it out.
Sometimes, when using Chewing Tobacco while riding a horse, a floating chair will appear where Marston's head was when he put the tobacco in.
Tobacco is something you chew then spit but Marston never seems to spit it out.