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FF0,FF1,FF2 are feed forward terms

FF0 adds a proportion of the PID command to the PID output
FF1 adds a proportion of the PID commands first derivative to the PID output
FF2 adds a proportion of the PID commands second derivative to the PID output

For a simple example imagine a spindle control PID loop
This would have a PID loop with velocity command and velocity output.
Setting FF0 to 1 means that at a say commanded RPM of 3000,  means
that the PID output would be 3000 without any correction from the PID error
term. You can think of the FF terms as a way of centering the operating
point of the PID loop around the current state (rather than having
feedback have to do all the heavy lifting)

 

No, it does not.
I scrolled through the HAl and could not find the hm2_7i80.0.encoder.00.input-index pin linked anywhere. So I assume Linuxcnc does not get this signal- can this be the reason?

If you set x-index-enable high manually, does it get cleared if you jog the axis? 
(enough for one full turn of the encoder)

Beware, this may trigger a following error

Works for me. I use the same potentiometer for jog and feed rate (changes are immediate during jogging and running commands):

I can set it high in HAL-show by pressing the SET button on the right side, but not by homing/tripping the switch.
Also with 1ms division I can not see the index rush by...

I was previously continually jogging by holding down the + or - buttons in Probe Basic. However now it only moves one step at a time. I’ve obviously made a change somewhere but don’t know where to look. 

Any ideas?

Thank you for your answer
I can change the speed with a potentiometer, but if I change it while the axes are moving, it doesn't take effect. I have to stop the axes, change the speed, and then move the axes again. What I want is to be able to change the speed while the axes are moving.

I'm going to run some tests this weekend.
To be honest, I'm having trouble understanding the purpose of the FF0, FF1, and FF2 terms. Are these offsets of the PID terms?

Are you saying you cannot set x-index-enable true in halshow?

hm2_7i80.0.encoder.00.input-index pin staying true may be OK as this just reflects its
polarity, and you are unlikely to catch the actual index with halscope because of its narrow width
(especially at 500 ms/division where halscope throws away most samples)

Unless you can set x-index-enable it will be difficult to diagnose