Polling catches most things, but iDRAC traps deliver hardware failures (a failed PSU, a degraded RAID member) the moment they happen instead of on the next poll cycle.
Under Settings > Sensors > (this sensor) > SNMP traps, turn on the trap listener; it listens on UDP/162 by default.
In iDRAC Settings > Network > Services > SNMP Agent > SNMP Alert, add the sensor's IP as a trap destination and set the trap community or SNMPv3 credentials to match what you configured on the agent side.
iDRAC's alert configuration page has a Send Test Alert button. Firing it should produce a detection in Linivo within seconds; if it doesn't arrive, check that UDP/162 isn't blocked between the iDRAC and the sensor.