The faces of Harley-Davidson cylinder heads, as with other engines, need to be both flat, and possess a good surface finish, in order for the head gaskets to seal correctly. This is even more important with compression increases and the use of Multi Layer Steel (MLS) gaskets. These MLS gaskets, whilst capable of withstanding very high cylinder pressures, are not very tolerant of distorted, or badly finished, surfaces. What the Americans call "decking the heads", is common practice in both engine remanufacturing, and performance upgrades, and machining larger amounts from the cylinder head "deck" is also an easy way to achieve a moderate compression increase.
Setting the quench area of the cylinder heads so that the pistons come to within .030" at Top Dead Centre (TDC) makes the engine more efficient and less prone to detonation, particularly important with higher compression ratios and the lower quality petrol that we have to endure these days. The Harley-Davidson Evolution Big Twin, Twin Cam, and the '04 up Sportster 1200 heads have a quench area designed into them. Unfortunately, these clearances are far too large on the mass produced factory engines, and aren't really effective.
What is often referred to as "squish correction" can sometimes be achieved with different thickness gaskets, or pistons (such as some of the Head Quarters ones), with a corrected compression height. However, where this cannot be done with available parts, we can face each cylinder head up to the point where the head gasket bore starts, and leave a short spigot protruding into this gasket bore. This brings the cylinder head quench area closer to the piston and allows it to do what it was designed for. Pioneered by Head Quarters in a Harley application, this is known as "Spin dropped heads".