I don'tthink it's that hard to understand....the guys themselves describe the defensive side pretty clearly, and I think the offensiveside is obvious. But just in case you need a hand...
Defensively - both are former All-defensive team selections. Both play bigger than their size on defense - they both can defend either guard position when healthy thanks to their combination of quickness, length, and strength. They can take turns pressuring the ball, allowing Boston to apply full court pressure for an entire game, whileeach ggy stillgets his chance to get a break.
The next time I see Rondo pressuring the ball would probably be the first time since his rookie year. Likewise, I've never seen the team apply full-court pressure for the whole game, and I think it's a terrible idea -- it's way too much energy to expend, it can be handled with a single back-court screen, and it's just not worth it to risk having your smallish guard run full speed into a 250 lbs guerilla while hustling up the floor. There is a reason why most teams in the NBA don't do that.
Oh, and it also means that one of those two will have to be on the floor at all times, which actually minimizes the gains from the alleged synergy (and may force Bradley in the PG spot at times, too).
Also, I don't think anyone has ever argued that Bradley plays bigger than his size on defense (as a matter of fact, he probably plays
smaller than his size because of his quickness, if that makes any sense).
Offensively - Rondo works best with the ball in his hands. He's a creator - a pass first PG, and a pure playmaker and floor general. He's most effective as a scorer when attacking the basket off the dribble. Bradley plays best off the ball - he's not a confident ball handler or passer. As a scorer he's most comfortable off back door cuts, off catch-and-shoot plays, and with a one dribble pull up. He's fairly effective with all of the above, and all benefit greatly from having a greatset-up guy who can get you the ball in the right spot.
Again, the fact that Bradley plays best when not forced into the PG spot isn't the same as saying that the Rondo-Bradley pairing is a great fit. A more conventional SG who can hit a three, shoot over defenders, and perhaps not completely fall apart if they have to finish a contested layup would be better. Rondo-Bradley wasn't a very good offensive pairing last year (Rondo-Lee, on the other hand, was).