Simul climbing or climbing with a running belay is a climbing

Rock climbing

Rock climbing also lightly called 'The Gravity Game', is a sport in which participants climb up, down or across natural rock formations or artificial rock walls. The goal is to reach the summit of a formation or the endpoint of a pre-defined route without falling...

 method or style

Climbing styles

Rock climbing style refers to the method by which vertical progress can be made in rock climbing. Each climbing style can be considered a sort of game with rules or standard commonly referred to as climbing ethic. These ethics are some of climbing's social mores.- Categories :The term "style" is...

 where both climbers, climb at the same time while tied into the rope. Protection is placed by a pseudo-leader and the second removes the pieces of gear. The length of rope used during simul climbing varies but is often between 15 and 30m. In most cases the climbing team maintain multiple pieces of protection

Protection (climbing)

To make climbing as safe as possible, most climbers use protection, a term used to describe the equipment used to prevent injury to themselves and others.-Types of climbing:...

 between them to prevent a system failure if one of the pieces was to fail. During true simul climbing neither partner uses a belay device

Belay device

Belay devices are mechanical pieces of climbing equipment used to control a rope during belaying. They are designed to improve belay safety for the climber by allowing the belayer to manage their duties with minimal physical effort. With the right belay device, a small, weak climber can easily...

. However at first the second may use a belay device until enough rope/protection is out for the leader to avoid a ground fall at which point the second to start climbing. Similarly the leader may use belay device as the second approaches a belay stations to avoid the potential for a large fall.

Fall potential

The roles of the leader and second in simul climbing are very different than they are in free climbing. If the leader falls during free climbing the second is pulled up from their belays station. In simul climbing, if the leader falls the second is pulled up from wherever they are climbing. These two situation contain a similar level of danger.

In free climbing the second is belaying and not in a position to fall while in simul climbing the second can fall. If the second falls while simul climbing the leader catches the fall with their own body. This jolt has a high likelihood of pulling leader off the rock towards their last piece of gear. This sort of fall has the potential to be devastating for the leader. To avoid this possibility its traditional to put the better climbers second so that they won't fall.

Variations

Simul climbing or variations of Simul climbing are commonly used while speed climbing

Speed climbing

Speed Climbing is climbing in which speed is the ultimate goal. Speed Climbing is done on rocks, walls and poles...

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