I don't really understand what the problem is, if a teacher instructs students to relax while doing the form, or before.
If I start doing tai chi with a tensed mind, then I usually also move more tense, whether I'm walking on a street or doing tai chi. With tensed mind, my body is tensed and not so willing to do things soft.
In a state like that, if I immidiately jump to do the form I most likely will do more damage than healing for myself. For example, I would take the first steps unrooted. And when not being rooted, I'm not relaxed...not being properly rooted means I must use unnecessary muscles and I can immidiately feel the tension starting from my feet, reflecting to my lower back, neck and head...not feeling good, and totally unrelaxed.
Sometimes again, being tensed...taking the first steps is the moment, where I'm suddenly able to redirect the energy coming from the tensed mind, to a more open minded state where softness beats tension and relaxation starts.
At other times I can't do that, for some reason. At those moments standing 10sek - 5min in wuji position forces me to confront the fears, tensions and other obstacles that prevent me from relaxing properly. At those moments an instructor who reminds me to "RELAX" might just be the key to soften up again.
"Don't do it if it hurts you." And what's wrong with this sentence? The first thing my teacher said to me is that when doing the form, don't hurt yourself. Taichichuan should make us healthier, I think the path to that is to learn what pain is good and what not, when one is relaxed and when not, discover the tensions building up the body when falling off from the center.
I might do same kind of exercises before I go for a walk. After all, how I train affects how I walk so I have to go through the same issues in everyday life, even now when I'm sitting.
I'd like to have an instructor right here beside me, watching as I type this text, and reminding me to sit properly and feel my shoulders relaxed.