Not sure it's a new tech I discovered while messing around, but it's pretty simple and self explanitory, you can jab out of a dash without any lag, without stopping your momentum and while keeping the direction of the dash holded

It only works on keyboard though, but is really simple and useful, to do it you simply have to:
- Dash in any direction, and keep the direction key held down
- Imput the opposite direction key (you can hold it during the whole dash it won't change momentum) without removing your finger from the first direction key
- Press attack


It really because useful when your character's dash attack can't jab reset and you have a decently fast character (for instance sonic who can f-smash into jab with this if the opponent misses the tech), or simply as a mixup for an offensive but safer out-of-dash (pikachu, zss or DK for instance are way safer with this than with their dash attack)

Big issue with that is it can't be done instantly, for some dark reasons you can't jab out of dash frame 1 you started it unlike tilts or smashes, there's a delay at the beginning where you can't do it (it will make sandbag do weird and random stuff btw if you spam it at the beginning for some reasons), you have to wait half a second before being able to do it, it kind of reduce the possibilities with it, but is still pretty cool to pull off.

I forgot:
You can also do it with neutral-special, and it can help you do a turnaround f-tilt out of a dash easier if you release the forward key right before pressing attack

What youre referring to is called Multidirectional Influence (or MDI for short). It has been around for quite a while now (sometime in 0.9b actually), but literally noone ever talks about it ever, so I see why you'd think this is a new tech. You can also do an MDI Nair to keep full momentum, which is really useful for most characters, and being a Chibi main, MDI Jab is my go-to Jab Reset option actually (also MDI Blaster is busted but thats besides the point). The delay you are experiencing at the beginning of the dash is the regular dash animation, during which you can only use dash attack, grab and JC USmash/UTilt if Im not mistaken. You have to wait until the initial dash is over and the regular running animation begins to get access to MDI related stuff.