Deh can mean both good or bad, like how aloha means both goodbye and hello. No one really knows where it came from. All we know is that it came from the depths of Snapchat.
(Scenario 1)
1) Oye, you deh? (Oye, are you there?)
2) Yeah mun, I reach (Yes bro, I have arrived)
(Scenario 2)
1) Oye, mate deh forming di ass! (Mate is making a fool of himself right there)
(Scenario 3)
1) He deh an' he not saying anyting (He's there and he's not saying anything)
(Scenario 4)
1) I asking her an' she deh like she doh understand what I'm saying (I'm asking her and she's standing there confused as if she's stupid and she don't know what I'm saying
"Deh pefforza" is an expression used and created by Volpescu(also known as Vompescu Twonch), an italian Twitch streamer. It is used for indicate something that will happen surely, or something that is undebatable for who is speaking.
It can be used also without a context, for strengthen a fact or whatever.