Dank is shorthand for something good.
Slap in this case is an affectionate pat as one would see a
baseball coach apply to the rear end of a player when they complete a good play. The two together is now a Dankslap.
Some
people see a Dankslap as an attempt to touch another person’s behind while hiding behind some type of celebration or support display when a person needs encouragement.
To express your support either positive or negative wait till the person is walking past you and then wind up and apply either a
single - almost too hard yet arguably not hostile slap or a couple
soft, quick pats to a person’s rear end while offering words of support - words are particularly important in order to establish you are trying to show support versus just copping a feel.