I believe it is caused by the changing of engione load durring a gear change? The difference will cause a change in pressure in the manifold especially on vehicles that close the throttle body durring a shift and those that have alot of lag between shifts. That is just speculation on my part though. If this proved true than decreasing the amount of l;ag between shifts could help keep the spikes lower and less in durration.

Anyone have facts to support or discredit this?

 

I think the pcm closes the throttle plate on the hemi's when shifting, causing the spike.

It was F8L that gave me the reference relocation idea many moons ago, and I and a ton of others thank him.