It’s pretty simple. Drum Key “Pan” and “Alternate Pan”

Pan
Sets the pan position (stereo position) of each Drum Key. This will also be used as the basic Pan position for the Alternate and Random settings.
Settings: L63 (far left) – C (center) – R63 (far right)

Alternate Pan
Determines the amount by which the sound of the selected Drum key is panned alternately left and right for each note you press. The Pan setting (referring to the Drum Key “Pan”) is used as the basic Pan position.

That basic Pan position can be thought of as the “Alternate Pan” parameter’s Center position reference.

Extra Credit:
First, the “Element Pan” must be ON for the Drum Kit Part
Then, in the individual “Drum Key” > “Level/Pan” area you determine how the alternate pan function will behave…

1__If you individually Pan a Drum Key to C, and set “Alternate Pan” = L64, the result will be the drum will toggle between all the way Left, and then hard right on the next hit. — this is likely what you are looking to do.

2__Same will happen if the Drum Key is C but the “Alternate Pan” = R63, the result will be the drum will toggle between all the way Right, and then hard left on the next hit.

These two examples will have the same response (with the exception of the direction of the initial strike… once you’ve got the hits going they are impossible to tell apart.

If however, you start with the Pan set to any value other than C, (as you did, you started Pan = an extreme Pan, R63 — which establishes a new reference point for C) then it will reduce the amount of distance it can move the alternate hit by half (you have established a new start point reference, a new Center, way over on the right side).

For example, if you individually Pan the Drum Key hard Right, R63, then set an “Alternate Pan” setting of L64, then it will toggle between hard Right and Center. Because your new Center reference is way over to the extreme right. To the machine hard left is 64 steps from the center reference point which you have now shifted to R63. The resulting alternate position is now equivalent to actual center.

If you individually Pan the Key to L64, then set Alternate Pan to R63, then it will toggle between hard left and Center. Again, the distance of movement is reduced by half.

Panning in a Stereo World…
Simple explanation of how panning works (may help): if you have a Stereo PIANO sample, to get each channel to sound in their respective channels you PAN that Piano to Center… left goes left, right goes right… the stereo illusion is maintained. Easy to understand

If, however, you take a Stereo piano sample but you pan it hard right, you will decrease it’s spread across the stereo panorama… the right channel is still hard right, but the left extreme is reduced by half… what was sounding all the way to left side is now heard from the center of the field. Meaning the distance from right-to-left has been reduced… it is only half the distance previously experienced.

I'm not sure I'd describe that as 'pretty simple', but your explanation is clear. I have now found it in the Synth Parameter Manual on pages 31 and 46 but not so clearly. Thank you!

I can't find a description in the manuals, though it must be in one somewhere.

For Drum Part Edit see the bottom of page 143 for Drum Key Pan and the top of page 144 for Drum Key Alternate Pan.

I thought it would make a drum sound appear at the set alternate pan position every other Note On event, but it seems to add or subtract the position instead

SPOT ON! That 'seems to add or subtract the position instead' is the key to understanding what Bad Mister explained.

ALTERNATE PAN is an OFFSET to the value you set for PAN. Move alternate pan 2 to the left and it will effectively subtract 2 from the Pan value you have set and so on.

I'm not sure I'd describe that as 'pretty simple',

Correct - but wait - it gets more complicated.

Like some other parms there is a HIERARCHY of Pan values that you can set:

1. Each Element/Drum Key has a PAN parameter

2. Each Part - common has a PAN parameter and also an ELEMENT PAN (ON/OFF) that determines
if the elements/drum key PAN values are offset or not

3. The Performance has a PAN parameter (see the common audio page)

So you have to apply ALL of the offsets in order to determine the overall effect.

The element level offsets for a part determine how the elements (or drum keys) of that part relate to each other (e.g. snare drum far left of the bass drum)

The part level offsets for a performance determine how the instruments (parts) relate to each other. For example is the drum section right or left of the bass section and piano/organ sections?

The performance level offset determine how the entire 'band' sounds to the listener. Is the band to the far left? the far right? Or dead center (you paid extra for those tickets!).

The following paraphrases what Bad Mister has said in the past (I hope I get it right):

Build your band from the ground up: instrument (piano, bass, snare drum, bass drum, etc). Then instrument group: horns to the left, drums to the right, violins in the center.

Then choose your seat: far left and in the back for the cheap seats. Up front in the middle for the most expensive seats when you want to impress someone.

Remember those two words: OFFSET and HIERARCHY.

The following paraphrases what Bad Mister has said in the past (I hope I get it right):

Not so much…