Note - This post discusses techniques and methods used in vertical rope work. If you do them wrong, you could die. Always practice vertical rope techniques under the supervision of an experienced climber, and ideally in a progression: from flat ground, to staircase, to vertical close to the ground before you ever try them in a real climbing situation.
Plaquette style belay devices like the Black Diamond ATC Guide and Petzl Reverso are great, allowing smooth rappels, to be rigged as an ascender, and to auto lock when belaying your second. But they do have one major drawback - if you need to lower your second, it's generally not safe, easy, nor intuitive to do so.
Here’s a Crafty Rope Trick (CRT) that's about the easiest method you’ll find to lower your second off of a plaquette. Note that this does not require any seldom used, hard-to-remember rescue geek knots like the Munter Mule Overhand (MMO), nor any sudden unweighting of the belay device that might cause your second to wish they wore their brown pants that day.
Plus, it has a great name - the “Load Strand Direct” lower.
Note: this method does require your follower to unweight the rope for a moment and give you a small bit of slack. In just about every possible climbing situation, this will be possible. Even so, I can already hear the peanut gallery out there, yelping “OMG, what if they’re unconscious! What if they fell on a traverse and are hanging out in space or on a 5.15 blank wall?” True, in those two scenarios this not going to work. However, those two situations are so incredibly unlikely to happen, it's certainly safe to learn this as your primary lowering technique. And, be smart about it - if your second might have a chance to swing out into space, you should probably be belaying with some other method all together, like with a Munter hitch, which is super easy to lower on anytime.
Okay, let's get to it.
Scenario: You’re belaying your second directly off the anchor from a plaquette style belay device. For whatever reason, they need to be lowered (after they’ve reached the top of the pitch, or from anywhere else.) Here's what you do.
1 - Tie a quick overhand or figure 8 on a bight as a backup knot in the brake strand of the rope.
2 - Put a prusik, autoblock or similar friction hitch on the brake strand, and clip it to your belay loop with a locking carabiner. This friction hitch will back up your brake hand when you start to lower in a moment. This is important, don't skip it.
3 - Untie your overhand or figure 8 backup knot.
4 - Clip a spare locking carabiner to the anchor master point, with the gate opening facing down. Note that this carabiner needs to be the same size or smaller than the carabiner that’s holding your belay device.
5 - When all this is set up and double checked, ask your second to unweight the rope for a moment. Doing this will cause a little slack in the load strand. Pull up this slack rope, and clip the load strand through the new carabiner on the master point.
6 - You’re now ready to lower. Slowly let out slack from the brake strand of the rope, backed up with the friction hitch. You should be able to lower your partner slowly and in control.
You definitely want to practice this in a controlled environment before doing it for real on the rock. There are lots of factors that can affect how fast the lower is, such as rope diameter, whether the rope sheath is new and slippery or old and stiff, the weight of your climber, the amount of rock/snow your rope is running through causing additional friction, the carabiners you’re using, and other variables. That's why you have the friction hitch backup; if the load starts to move too quickly and your palms are smoking, that hitch is there to stop things in a moment.
Now check out this nice video to really learn it, direct from Karsten with Fox Mountain Guides.