Question about backup shard cluster using ops manager&mongodb agent

i have a question about using ops manager to backup shard cluster:
Does mongodb agent can only be active in only on host?
then how ops backup the whole shard cluster if the host (which has active agent ) does not have all shard members.
for example: 4 hosts with three shards ,every shard has 3 replications.
host1 host2 host3 host4
shard01_primary shard02_primary shard03_primary shard04_primary
shard02_second1 shard01_second1 shard04_second1 shard03_second1
shard03_second2 shard04_second2 shard01_second2 shard02_second2

then i installed mongodb agent in every host, and only agent in host1 is active, but host1 does not have data of shard04, how ops manager backups the whole shard cluster data?

i read the document :

it says:
Only one host can backup a deployment at a time. On the Server tab, the host that is backing up the deployment displays Backup - active. Any other host with Backup activated displays Backup - standby.