Shutdown for me, is totally flat affect, zero eye contact, and scripted replies, if I talk at all. I literally switch off every vital aspect of myself as a person. You want an autistic robot stereotype, my shutdown mode is it.
If I ever take my glasses off, it is a red flag that I am in overwhelm heading into shutdown. I cannot see anything six inches beyond my nose without my glasses. I need them to function; I take them off and it blurs the incoming sensory stimuli and removes the plethora of details, I would otherwise compile.
Shutdown is a short duration switch off. Meltdown is an overload fueled outburst of emotions.