A backyard ultra is a format of ultra running.

Competitors  are to complete a 6.71 km lap every hour on the hour.

If a competitor  fails  to complete  the lap within the hour or they don't  start the next lap they are out . The race continues  until  there is only  one competitor  left and they are deemed  the winner.

Any spare time after finishing  the lap is time that a runner can use to rest and recover  and prepare  for the next lap. 

For the fund-raiser Chris Martin is going to complete an artwork  in the rest periods between laps.