I'd heard about this a while ago and checked in a few gears to confirm my suspicion regarding the cause. Going down the road hard on the gas then let off, then back hard on it. Keeping a finger on the lever I can feel the cable pull on the lever when I get on the gas and the cable go slack(er) when I let off. I checked in 6th. No change in the lever position during transitions. 5th gear. No change. 4th gear. Change. I may have tried 3rd and noted change. I forget.

At any rate, I think what you're seeing is the result of the quieter 2nd, 3rd, and 4th gearsets. The helical cut gears are integral with the transmission mainshaft. When on the throttle in any of those gears the gear cuts cause the shaft to try to screw toward the right side of the engine. That's where the cable interfaces with the pushrod going through the mainshaft across to the throwout assembly in the clutch. When all the slack is taken up in the bearings the pushrod is pressing against the cable-end actuator which causes it to take up slack in the cable. When you release the pressure on the mainshaft by letting off the throttle it lets off on the pressure against the pushrod actuator and the cable goes slack(er).

The only helical cut gearsets are 2, 3, and 4 (at least the first half of their reductions). 1st is also integral with the mainshaft but it's a straight cut gear so it won't cause the shaft to work sideways. Same for 5th (even though it wouldn't cause side-shift anyway since it's not attached to the mainshaft). 6th really ain't any gear at all, it's merely the mainshaft being tied directly to the output sprocket.

So, check this anomaly carefully. Does it really do it in any gear? Or does it mainly do it in 2nd, 3rd, and 4th? (all the gearsets are meshing all the time so there'll be some thrust on the mainshaft all the time but it'll be proportional to the power being delivered through the gearset(s); that is to say that the lever may detectably move in every gear but not like it does in 2-4)

Just don't tension the cable too much and you'll be okay.