Understanding Secondary Log Hard Disk Impact on Write Speed and Data Integrity After Power Outage

I've recently come across the notion that a secondary log hard disk can be employed to enhance write operation speed in a database system. I find this concept intriguing but am struggling to grasp the mechanics behind it. Could someone shed light on why a secondary log hard disk is effective in accelerating write operations? Moreover, I've stumbled upon the statement that both SSDs and HDDs can retain all data written, even after a power outage. This caught my attention, and I'm curious to understand the mechanisms that allow both storage types to maintain data integrity post-power outage. If anyone could provide insights into this aspect as well, it would greatly contribute to my understanding.