Timing valleys

[robof16]

Did some searching and haven’t found anything…
Always wondered why the timing tables have the distinctive valleys in them. I expected the timing to be somewhat linear as RPM increases.
Any insight?

[killerbee]

valleys exist where transient "load" is typically being increased by the operator with his foot. Any time load is higher, the auto-ignition begins earlier and progresses with more speed, hence a timing retard is called for.