Skip the carabiner in the top jumar hole
A lot of beginner aid climbers want to add a “backup” carabiner to the top hole in their ascender, all the time. It’s not wrong, but usually not optimal. (Note: a locking oval carabiner works well here.)
I get it. The first few times you use ascenders it’s pretty scary, and you probably want every possible shred of psychological safety. If that carabiner on top can lessen the chance of the ascender coming off by 0.01%, that is a good thing, the reasoning goes.
Here’s the main reason why you usually do NOT want a carabiner in the hole. When you clean a traversing pitch, you probably will remove the top ascender and leapfrog it past the piece of gear you want to clean. To do this, you need to remove and re-attach that carabiner every time, which is a major time waster if you have a more than a few pieces to clean. Now, if you’re cleaning an absolutely vertical crack, then sure, add a carabiner in that hole if it makes you feel better, because you won't need to leapfrog it past any gear. But such a crack is the exception, not the rule.
You only really may need this backup carabiner if the rope is stretched tight and you can’t tie backup knots, and/or if you’re cleaning a roof, and/or if the rope is running at an angle greater than about 45 degrees off to either side. In other words, some oddball rope angles not encountered on a normal pitch of cleaning.
If you’re worried about the ascender popping off, remember this. If it does, you have your second ascender, and hopefully the rope running to your Grigri and/or tying backup knots as a third point of contact.
Try this - set up a tensioned horizontal fixed line, clip an ascender to it, and try to torque it in every crazy way you can think of to get it to pop off. It’s darn hard to do it! Not to say it can’t happen, but it may be more secure than you think.
Also, ascenders usually pop off the rope due to user error. After you remove the top one and leapfrog it past a piece, it's a common mistake to not click the toothed cam back properly to the rope. Always be sure the ascender fully clicks closed onto the rope every time you put it on, especially if the rope is under tension in a weird direction.
So, it’s best practice, most of the time, to not clip a carabiner through the top hole. If you do, it’s probably best if you use an oval carabiner. Also, be sure you clip it correctly around the rope, and not in the completely incorrect position shown below!
Be sure you don’t clip the hole like the photo on the right! At first glance it appears to be clipped correctly, and will probably hold your weight. But only a small part of the cam is engaged on the rope.
If you’re ascending a severely traversing pitch, it’s probably a better plan to clip the locker that’s connecting your tether to the ascender to the rope with spare carabiner, something like this:
Doing this can help keep the ascender more in proper line with the rope. (Aider and locking carabiner not shown.)
Finally, it's common when big wall cleaning to tie back up knots and clip them to your belay loop. This does a couple of things. It's a safety system in case your ascenders for some crazy reason fail, and it also helps with rope management, keeping the rope fairly close to you so it doesn't blow around and get stuck.
