String-bet question - what is the rule?

[LuckyTxGuy]

Playing $4/$8 at a B&M I frequent, I'm in the 1 seat and it's my turn to act pre-flop. I have both hands hanging over the rail, one on each side of my chip stack. In each hand I have $4. I glance down, make sure each stack is $4 and without (as I always do) saying "raise", in one fluid motion, I put both hands across the line and into the pot. I set both stacks down side by side for my $4 call and $4 raise. The dealer immediately says "string bet, that's just a call, I watched you and couldn't believe you were really about to do that....". Stunned I said ok and let it go but I don't understand how that's a string bet?

1) If I make one fluid motion with chips across the line with both hands at the exact same time, how is that a string bet? Or is there some rule that says you can only raise with chips in one hand? I've searched online, read Robert's Rules and no where can I find the rule that says this. I understand how it could be construed as a string bet if I put one hand across the line, then the other one followed a second later. However, that is not the case, I'm talking two hands, side by side, one fluid motion forward across the line in unison.

2) That being said, let me ask this..... if that rule is correct does that mean that I could push both hands forward at the same time, just like in the scenario above but only take one hand across the line with the $4 call and stop the other hand just short of the line while flicking my wrist towards the pot (like I'm intending to throw the chips in) and I wouldn't be forced to raise? That seems like that would be an angle shoot to me and a case to force the bet because of the forward motion.

The hand wasn't a big deal at all and I didn't make any issue of it at the time. I always say bet, raise, call etc and I should have said it that time, but I'd really like to know what the rule is here and why.

Thanks