ReverseTraffic switches traffic in the reverse direction for the tablet_types specified. The traffic should have been previously switched forward using SwitchTraffic for the cells/tablet_types specified.
For primary tablets, ReverseTraffic first stops writes on the target primary and waits for the replication on the source to catchup with the gtid on the target where the writes were stopped. If the wait time is longer than timeout the command will error out. For setups with high write qps you may need to increase this value.