You only have to move the A/T switch to T (time) for long exposures, beyond the shutter speed range of the camera. The switch is moved before exposure, and when the shutter release is pressed, the shutter will open and stay open until the A/T switch is moved back to A. To avoid any vibration affecting the shot, a lens cap can be carefully put on, or something opaque can be held over the front of the lens while moving the switch back to A.
The range on the shutter dial goes to 8 seconds, but there is a B (bulb) setting on the ETRSi only. The only way to manually take exposures beyond 8 seconds on the older models is to use the A/T switch. The B setting on the ETRSi uses battery power the whole time the shutter is open, so Bronica recommends not using B for more than 1 minute.