These days many instruments are software based. This isn't only true for software MIDI instruments like the Sampler or VSTi plugins, but for guitar amp simulators like the Guitar Amp effect as well. Practice Mode lets you use these instruments without actually recording them, so you can practice your part or use a keyboard to figure out chords etc.
Manual Practice ModeYou can switch a track to Practice Mode by doubleclicking its Play button. The button will read 'P' while the track is in Practice Mode. The track will stay in Practice Mode until you click either its Play or Rec button.
Note that Soft Monitoring must be used for audio tracks to be audible.
Automatic Practice ModeA track will be switched to Practice Mode automatically if some conditions are met. A MIDI track will be switched to Practice Mode if the MIDI instrument's window is visible at the time the transport starts while neither the track's Play button nor the Rec button are "on". Similarly, an audio track will be switched to Practice Mode if one of the effect windows is visible at the time the transport starts while neither the track's Play button nor the Rec button are "on".
Automatic Practice Mode also works while the transport is stopped, regardless of the track's Play/Rec buttons. Alternatively you can engage a track's Rec button, in this case there's no need for any effect or instrument window to be visible.