A descender or rappel device is a friction device or friction hitch that allows rope to be paid out in a controlled fashion with a minimum effort to the person.
If you have never abseiled before start on the ground or shallow slope before doing it for real. Learn and practice before you abseil down popular route in the middle of summer.
When using natural protection (trees, boulders, or spikes) make sure they are absolutely solid. Anything that is not permanently attached to the mother Earth should be treated with suspicion. Always make sure that your anchor is bombproof and safe because you might only get one chance to abseil from old dead rotten tree or loose piece of rock. Descend should be smooth and slow. Don’t bounce and avoid sudden drops as this can shock the anchor in same fashion as when you jump on your bathroom scales and they go way off the scale…
If you use in situ abseil point always check all the equipment left in place. Look for signs of corrosion and overuse or damage to slings, bolts and tats. If you are not satisfied use your equipment and leave it behind. It’s preferred to replace sling to losing a life.
When preparing for an abseil always look over the cliff edge to make sure nobody is below. With one end of the rope attached, pile rest of the rope into neat pile, and then holding last few meters in coils throw them out and down. Shout “Rope below!” to warn everyone. Check that rope reaches the ground and is not stuck to anything on cliff face or trees sticking out. Attach you belay device to the belay loop on your harness and feed the rope through same as when belaying. With both hands on the braking rope lock off and adopt same body position as when being lowered of the route. Let the rope slide slowly through your rope. Never do that SAS squad abseiling you have seen on Macgyver, unless you want to see a red hot belay plate burning through your new shiny nylon rope. Always descend in a control manner!
How can you make abseiling safer? When abseiling you can slip and let go of the rope. This could have drastic consequences. It’s good idea to use auto block as a backup to hold controlling rope in case you let go. Widely used auto-block is French Prussic using thinner loop of rope (6-7mm). To make French prussic wrap 4-6 times around the rope and clip booth loose ends with carabineer and attach this to the leg loop on your harness. Make sure that carabineer is clipped on the inside of the leg loop as clipping to the outside can cause buckle to come undone as the carabineer hits the buckle and can hold the buckle open. Now you have a backup and in case of losing control over your descent prussic will bite the rope and stop your descent. Remember prussic is not fail-safe. It could rub against something and release. Make sure prussic don’t get too close to your abseil device as it could get jammed inside and you will not be able to get it out of device while there is a weight on the rope or it could completely open resulting in a fall. One way around this is to extend the abseil device further from your harness using 15 cm quick draw or a short sling. In case you want to stop while ascending allow prussic to bite slowly onto the rope and wrap 1 or 2 loops of control rope first around your waist then around your leg. This will ensure that you will not descent and that you will not turn upside down. If you are abseiling on both halfs of single rope or double rope tie knots on the end. This will prevent from slipping of the rope and fall.
Simple abseiling set-up with double ropes (don't forget to learn in controlled environment)
And with safety a break:
Another way of setting up abseil. Note that abseil device was extended to prevent prussic getting stuck or pushed open.