The benefits of chunking down a difficult climb into parts are both mental and physical. Psychologically, it reduces the burden of a long hard route by allowing you to consider its parts as several shorter, doable climbs. Physically, chunking lets you dedicate full energy to solving the crux chunk first, as if it were a route of its own. Only when this is sent, do you begin work on other “easier” parts of the climb.
Before you can chunk down the route, you need first-hand knowledge. Take a quick “reconnaissance run” up the climb to determine its logical parts–how you chunk down is, of course, route dependent. For instance, a route made up of 70 feet of moderate climbing followed by just 10 hard feet to the top is best chunked in two parts. Knowing that the first 70 feet is doable, get to work on solving and linking the last 10 feet while you’re fresh.
On a climb with multiple cruxes, chunk down each difficult section into a “route” of its own defined by good rest positions or gear placements. Sport climbers commonly break down routes bolt by bolt (such as, a 10-bolt climb having 10 chunks). After the first “run” through the route, grade each part (such as, hardest, second-hardest, easiest, and so forth) in your mind considering that the upper parts may seem harder on redpoint due to fatigue. Always make solving the hardest part top priority.
The most popular method of “linkage” is to climb ever-increasing lengths of the route to the top. Consider the common scenario of working a route with the rope already through the top anchors. From the anchors, lower down the route (backclip if overhanging) only as far as you think you can “redpoint” back to the top. This might be only a 10-foot chunk if the end is the crux. If you succeed at that chunk, lower down and add another chunk to the linkup. Continue adding chunks until your starting point hits “easy ground” or the ground. This process of linking chucks can take hours or days depending on the length and difficulty of your project. But there’s much greater value in this approach compared to the old start-from-the-ground method.
Linking from top down makes you extremely familiar and confident in the final portion. On redpoint, it’s here you will want to be super dialed-in due to the accumulation of mental and physical fatigue. Oppositely, the from-the-ground approach commonly leaves the climber thrashing to refigure the top part, which he has practiced much less. Such ground-up efforts are more likely to result in a fall off the final moves, making a waste of even the most heroic effort.