I an an MCI and was in charge of a group of 4 pupils who were all experienced climbers and had been "signed off" as independent climbers at Perth Climbing Wall. In the second half of the session, the climbers split into pairs to practice "lead climbing," which involves clipping in as the climber ascends. As there was a weight difference between XXX and his belayer (YYY), a secondary device called an Ohm had been fitted to the first bolt to add friction to the system and prevent the belayer being pulled off the ground. The belay device used was a Grigri 2. Both devices were correctly fitted (this was confirmed by the Senior Instructor at Perth Wall, Eddie Baker). As XXX was attempting to clip the final top anchor point, he was starting to run out of strength. This was being filmed at the time by Mr Jeffers for coaching purposes. I was not concerned by the prospect of a fall, as there was plenty of rope to absorb the fall, and there should have been no prospect of him reaching the ground. At this point, XXX did fall as anticipated but the fall was not stopped, and he struck the ground. At this point, help was immediately given and XXX was told to lie still and an ambulance was called. In order to facilitate this, I removed the rope from his harness. The knot was actually significantly difficult to untie and evidence that the rope had in fact been loaded and absorbed a lot of the energy of the fall. The paramedics could find no obvious signs of injury, but he was sent to Ninewells for an X-ray as is standard practice in these cases. The Dr examining him at A&E could also find no signs of injury, but he was sent for an X-ray, which confirmed that there was no damage to any bones. The X-ray was also checked by the on-call Radiologist, and XXX was discharged. The incident has already been reported to the Executive Officer at Mountain Training Scotland as they have oversight of all climbing-related accidents and near misses and share any lessons learned within the climbing community. It is the opinion of Mr Jeffers that the presence of the Ohm may have reduced the speed of the fall to below the threshold needed to initiate the assisted braking function of the Grigri. This should have been prevented by the correct belay technique of always keeping a firm grip on the "dead" rope. In conversation with the belayer, it seems he was holding the rope but did not grasp it firmly enough to prevent it from running through his hand. The primary lesson at this stage is to constantly reiterate the need to keep a firm grip on the dead rope at all times, but advice will also be sought on how the introduction of the Ohm device might have affected the grigri. Despite this, both devices introduced sufficient friction to slow down the descent sufficiently to prevent serious injury.