I think I've used the cue 'get deep' in two ways:
1) training oppo/deep off tee or soft toss helps to develop a proper bat path covering all contact locations (as does training up and down in the zone), and..
2) against live pitching using it to talk about how to 'go with' or hit oppo on an outside pitch. With the fix being if you get out early on that pitch it's an almost guaranteed groundball out. Understanding how a pitcher is trying to get you out via pitch location and having an answer to that results in more hard-hit balls and using the whole field....
1) training oppo/deep off tee or soft toss helps to develop a proper bat path covering all contact locations (as does training up and down in the zone), and..
2) against live pitching using it to talk about how to 'go with' or hit oppo on an outside pitch. With the fix being if you get out early on that pitch it's an almost guaranteed groundball out. Understanding how a pitcher is trying to get you out via pitch location and having an answer to that results in more hard-hit balls and using the whole field....