Bulk walk snmpv2 monitoring


We are running the CheckMK RAW edition (2.1.0p9) and have the default “Enable SNMPv2c and bulk walk for hosts” rule enabled so if all is well, our CheckMK server uses bulk walk for all hosts we monitor.
Some things, however, remain unclear to me. CheckMK has two SNMP services/active checks per host (Check_MK and Check_MK discovery). If I understand correctly, the Check_MK discovery service will periodically check the hosts for new services/interfaces. The regular Check_MK service only checks the status of the monitored services, is that correct?

If the information above is correct, how exactly does the Check_MK service/check work? Say we have a switch for which we only monitor 5 services (3 interfaces, CPU and memory), does Check_MK just bulk walk for get the status for just these 5 SNMP OIDs?