Why would threading a sewing machine with the foot down cause faulty stitching (and what about threading with the foot removed)?

Question

In trying to troubleshoot some bunched up thread on the back of the workpiece, I noticed the following wording in the manual of my Bernina Activa 145:

Faulty stitching... Sewing computer threaded with foot down

Why would this be the case? Obviously the needle position might have an effect, but raising/lowering the foot is a manual operation and independent of the needle.

As it's far more ergonomic to thread the needle with the foot out of the way of my (rather large) thumb, would it make a difference to remove the foot entirely for threading, with the foot holder in the lowered position?

In general this section seems poorly written, in that it refers to parts of the machine that aren't mentioned anywhere else in the manual and not shown in the diagrams. But the bullet point is unambiguous.

Answer 1

In all common sewing machines the tension disks are always engaged when the sewing foot is lowered. This may cause problems during threading if the thread doesn't properly slip between the disks (so it runs over them without actually being tensioned) or even slips from them entirely. I once had this happen to me and it caused long loops of the upper thread beneath the fabric because whenever the little arm thing that pulls the thread back up during each stitch pulled, it pulled more thread from the spool instead of closing the thread loop beneath the fabric.

Btw, the same can happen if you sew with the foot in the raised position, because that also disengages the tension.