That's totally true; flank steak has long, fibrous grain and the marination breaks down the connective tissue and when cut at a severe angle and across the grain, it turns a blah piece of meat into a delicasy.
From Wikipedia:London broil
is a North American beef
-based food dish usually made by broiling
or grilling marinated flank
or round steak
and then cutting it against the grain into thin strips. The origin of the name is obscure; the food is unknown in London, England
Although many American butchers will label a cut of meat "London broil", the term does not refer to a specific cut. The cut of meat traditionally used is flank steak, but top round roast/steak is also commonly used. Because the muscle fibers run the entire length of these cuts, the meat can be tough if not tenderized
via pounding or massaging. Scoring, stabbing, cutting, penetrating, or otherwise mutilating the cut before sending it into the broiler results in a tougher finished product as it allows all the desirable juices to run out of the meat into the pan.
The preparation of London broil typically involves marinating the meat for several hours followed by heating in an oven broiler or outdoor grill. In both heating methods the meat is placed approximately three inches from a direct heat source and turned several times to promote even cooking and avoid burning. It is commonly served in thin slices, cut across the grain.
In Canada a ground meat patty wrapped in flank or round steak is known as a London broil. Some butchers will wrap the flank steak around a concoction of seasoned and ground or tenderized flank steak (Zehrs Grocers in the GTA). Others sell a pork sausage patty wrapped in flank or top round steak labeled as London broil (Goeman's Lakeshore Meats in St. Catharines, Ontario, Canada). The website for Goeman's 
differentiates between Canadian London broil (the sausage patties wrapped in Top Round Steak) and American London broil (Top Round Steak). Another variant, popularized in Ontario, is a London broil "loaf", wherein the tenderized flank steak exterior is wrapped around minced and spiced veal as the filler. In some regions, bacon will be added between the flank steak and the veal grind.
Alot of good ideas there!