The idea or principle of Form truly is an amazing thing when used naturally and explored as an idea or principle. However when the IDEA of the thing is taken and turned into a dogma or law adhered to without consideration, this can really cause problems for the Systema practitioner stunting natural movement.
Form is so much more than inflexibly walking around, and transitioning from standing to the ground and up again with a rigid straight back all the time.
Good posture can certainly be the start of good form. But it's not all there is to it.
Good form also allows you to distribute whatever natural level of tension that resides in your physical being evenly and proportionately throughout your physical being. This combined with the integration of continuous movement and use of correct breath allows you to function in the most efficient and least tiring manner, thus giving you the appropriate level of relaxation, using just the right amount of tension necessary to perform a task without resorting to excess tension.
With good form you are way you are able to move naturally never occupying the same space and constantly changing shape (form) whilst still being able to maintain your structural integrity. Sensitivity and awareness of the body is key
As there are many different people with different body types, levels of tension and movement, perhaps a better way at looking at form rather than simply as "absolutely good or bad", would be to see the idea of form as a CONTINUUM, to be constantly observed in fluctuation and worked on by the individual on a daily basis.
There are degrees of good form and bad form. It's really amazing how we fluctuate along this scale from day to day.
The Work? Move baby. And pay attention to yourself
Systema Sydney Russian Martial Art