For me, part of dealing with PTSD is that I can easily find myself feeling unsafe even in the "safest" of locations if memories or sensations are somehow recalled or triggered by how I am experiencing what is going on around me. A good therapist realizes that and keeps check of how a client is feeling internally because lapsing into dissociation can be traumatizing in itself. My therapists were very good at sensing when I was on the verge of dissociation and knew by checking in with me about how I was feeling, how safe I was feeling, they could help me stay more present and our sessions were more productive.