The dynamic belay
First of all, just to clear-up a few myths, the following is not dynamic belaying:
1) Dynamic belaying involves giving lots of slack - FALSE!
The more slack there is in the system, the further the climber will fall before the rope can start to do its job and the more force will need to be absorbed. Unless there is an obvious hazard that you need to steer the falling climber away from, give only enough slack to allow freedom of movement.
2) Dynamic belaying is about letting the rope slide through the belay device a bit - FALSE!
Arresting a fall requires an almost instinctive response, there isn't enough time for the fine motor skill required to allow for controlled rope slippage, the risk is you will drop them altogether (note: this can be done but requires gloves, a figure of eight as a belay device, and preferably a back-up belayer.)
The principle shock absorbing element in the system is your rope. The more rope that is available to absorb the fall, the softer the fall will be. It is also worth noting that thinner ropes stretch more than thicker ones. You might feel very safe at the first bolt when clipped into a nice fat 11mm rope, and less safe when sixty meters up a pitch with a skinny 9.7mm winding its way back to your distant belayer, however, counter to intuition, the opposite is true.
After a rope has taken a fall, a degree of its elasticity will have been temporarily compromised, and it is wise to switch to using the other end ñ or another rope if both ends have been fallen on.
How to give a dynamic belay
A dynamic belay is dynamic because the belayer moves. How it is given depends entirely on the weight difference between the belayer and the falling climber.
A lighter belayer will naturally give a dynamic belay because they are automatically pulled into the air.
A heavier belayer needs to be more alert and should aim to adopt a position a couple of meters away from the base of the route - 1. To dynamically hold a fall, the belayer must anticipate the split-second before the rope goes tight and at that moment, lock off the belay device and move quickly to the base of the route - 2. As the belayer moves, the rope will be tight, but the full force will have more time to be dissipated, resulting in a soft fall with less risk of slamming - 3.
A dynamic belay will result in the climber falling further than they would otherwise. The important thing is that the fall is arrested slowly, not that the distance of the fall is minimised.
One way of giving a dynamic belay is to walk in to the base of the route as the fall is being arrested. It can be very effective, but the technique poses the risk of slamming the belayer into the wall - and should not be used where the belayer is lighter than the climber.
The crouching method
A better method is to crouch down and stand as the force is transmitted. To do this properly you must be in the act of standing up as the force is applied - standing after the impact is felt is too late. There is no need to crouch the whole time - just when a fall is most anticipated or when there’s not much rope out. If you are paying attention, you should be able to crouch down as the climber falls, then return (aided by the pull of the rope) to a standing position as the force is transmitted.
Avoiding creating hazards
For the first clip it is important to keep the rope out of your partner’s way. Stand too far out and your partner could easily fall onto the rope with painful consequences in the groin region! One way around this is to stand (or, better, crouch) to the side of the first clip allowing a degree of dynamism in the belay whilst keeping the rope out of the way. Once above the third clip, you can move out from the base to get a better view of what is going on.
Lastly, be sure that you first clear obstacles like sacks out of the way. The last thing you want to do is trip over when you’re belaying.
If a Grigri is being used to belay with, the problem with paying out slack quickly without letting go of the dead rope can be solved by standing a few metres away from the base of the route. When slack is urgently required the belayer can simply walk into the base, immediately freeing up slack. The belayer can then pay out slack at a more leisurely pace whilst walking back into their position away from the base of the route.
Sport CLIMBING + Second Edition
The Rockfax book Sport Climbing + includes many more useful tips like this which will help you climb better and safer. For example, have you ever wished to know how to get down from mid-route by abseiling off a single bolt without threading it or losing any gear?
The book is not just about rope techniques though. There are chapters on technique, onsighting, redpointing, the mind, training, mutli-pitching, self-care and destinations.