bind <key> scroll_backward

This executes the WINDOW SCROLL_BACKWARD command. It moves the scrollback view backward, towards older output. The amount it moves is determined by set scrollback_ratio as a percentage. It is normal for the client to switch to the scrollback view when you scroll back, so you should immediately see an effect.

When you reach reach the start of the scrollback buffer, further attempts to go backward will beep, and the scrollback will not change.

Whenever the scrollback view is active, new output to your window is buffered (“held”) without limit, so you will never lose any output. A common cause of high memory usage is a window on a busy channel being stuck in scrollback mode and then forgotten. Since the scrollback is never trimmed, it grows without limit, which can use up a lot of memory quickly. You can use the %K status bar expando to tell you when a window is using the scrollback view.