There are two majorly negative scenarios you will introduce by using an extension. The first is canyoneering-specific awkward rappel starts. There are many anchors that require you to pull rope through your device tight and practically draw your waist/device right up to the quicklink in order to put your bodyweight on the anchor, then scoot/swing into space above the drop. Having an extension means you can pull the device tight against the quicklink but you may not have the ability to weight the extension from a comfortable position. I watched a guy struggle with this, he pretty much had to hang from his off-hand on the sling (keeping brake hand on) until he weighted the rope. Another time he essentially scooted into a slide off the edge until the extension jolted tight, (i.e. needless shock loads). The second negative scenario is a mid-rappel transition over an edge i.e. from a slabby rappel into an undercut free hang. When your device is close in to the body, you can usually focus on negotiating your body around the corner and the device follows in your "wake" like a boat, it clears the edge by the time your body is over it. I've seen a guy stuck hanging on his extension for a good dozen seconds in the below scenario: It took punishing the critr, the rope, and his fingers and some mid air flail to launch himself weightless for a second to get the critr around the edge (another needless and graceless shock load) Hank Moon touches on the fact over in the near misses subforum that the instinct to grasp the rope will defeat the autoblock. To engage the block, you have to do 1 of 2 options: either purposely slide your hand up into the ATC to let the block fully stretch out, or let go of the brake strand to grasp it below the block. You are already losing control, so letting go is instinctually not possible, and having the mental clarity to move your hand forward while panicked probably isn't happening either. A prusik above the device will see full bodyweight and a corresponding higher level of friction. Climbers will rightly balk at the real possibility of melting a nylon prusik in this orientation. The Bluewater VT Prusik is made of Technora and will not melt. You can apply the higher friction and it will not melt. Tied in the valdotain-tresse it can be released weighted with bodyweight. It needs the room above the device to stretch out. When the device is extended out and your hands are close in, you gotta add an extra level of attention to not pull your hands in close enough to cause the rope to abrade the nylon sling. A final critical thought: I have yet to see someone stop mid-rappel and perform a critr-style horn wrap lock-off while using an extension and autoblock. If you video yourself doing this and post proof, I will buy you a beer. What, then, is the best practice to protect the first man down? 90% of the solution is choosing a first man who habitually and regularly uses multiple friction settings. If you choose a first man or woman who can answer yes to "do you use different friction settings on practically every rappel" then you can rest assured he or she will have the practiced habitual ability to add friction when needed. Wrong answers include "what's a friction setting" and "What? Don't worry I got this bro" . If he or she casually says things like "well I really like to take my other hand around and get two braking hands once I set my friction" your face should be like and practically kick this person over the edge. Now that you found a practiced and habitual friction setting user, adding a prusik backup is his call to make, but you already covered the most critical part.