LAPD dispatchers have no ability to shut a repeater off, and the dispatch
frequencies are set up only for use in duplex (repeater) mode anyway. Nor can they monitor or transmit on the division "simplex" frequencies. If officers have confidential information to pass along, they would send it via their MDC or else use a telephone to call whoever they need to notify. Patrol units, and most others, don't have encryption enabled. Yet.
As xts3000r said, "Toggle B" refers to the officers switching their radios to simplex mode for unit-to-unit communications on a tactical frequency.
The two common reasons you'll hear only one side of a conversation are
(a) In "Toggle B" mode (true simplex) if one unit is close enough for you to hear but the other one isn't; or else
(b) if one officer stayed on the repeater but the other one went to Toggle B (true simplex) and is too far away from you.
With LAPD's system, there is really no reason to switch your scanner to the input frequency, because whenever someone transmits on there it gets repeated.
Originally Posted by XTS3000
Most likely what's happening when they say toggle B, is the repeater is temporarily shut off and the dispatcher is listening to the input of the repeater. They do this all the time when the officer wants to say something, but doesn't want it broadcast to anyone else. Usually it's more personal information being said when this happens.
So find the input frequency to the repeater and when they say switch to toggle B, listen to the INPUT frequency of the repeater. You'll have to toggle back and forth between the repeater and input freq to hear everything as long as your in range of the radio transmitting on the input freq you'll hear everything.