Still, I don't see this as a string bet, since as I recall a string bet is defined as going back to your stack for more chips. DN picked up chips in both hands, then in a forward motion with both hands dropped chips from one, then the other, while also making his "pound you back" comment -- I don't see how that could be considered anything but an intended raise.
There are three ways to make a valid raise:
1 - Without saying anything, and in a single motion, put out the exact amount of chips that constitute the call and your raise. Note that this can't be done with a single oversize chip (putting out a single 1000 chip, intending to raise to 1000, when the bet to you is 350 for example) as it'll be ruled a call.
2 - Verbally announce that you're raising, put out the amount required for the call, then in a single subsequent motion put out the amount you're raising by.
3 - Verbally announce that you're raising, and state the amount you're raising too. You're then obliged to raise to that amount, but you can return to your stack as many times as you need to in order to complete the bet.
This is why there's an argument over whether "pound you back" counts as Negreanu verbally declaring that he's raising: if it counts as a declaration then his bet falls into the second category and it's not a string bet. If it
doesn't constitute a verbal declaration, however, then it falls into the first category and it's a clear string bet.
You don't have to go back to your stack for it to be a string bet, you just need to make the bet in multiple motions. Like I said above, pros do it all the time, they're just rarely called up on it.