Short for "simultaneous", simul climbing is when two climbers move at the same time. The pseudo-lead climber places gear that the pseudo-follower collects. When the leader runs low on gear they construct a belay station where the follower can join them to exchange gear. The stronger climber is often the pseudo-follower since a fall by the follower would pull the leader from below towards the last piece of gear—a potentially devastating fall for the leader. In contrast, a fall from the leader would pull the follower from above, resulting in a less serious fall. Most speed ascents involve some form of simul climbing but may also include sections of standard free climbing and the use of placed gear for advancement (i.e. partial aid or pulling on gear).
Is simul climbing more dangerous for the leader or the follower?
Simul climbing is more dangerous for the leader, since a fall by the follower would pull the leader from below towards the last piece of gear.