I really am curious as to what the super secret mystery skills are that no one else in the diving world can know without GUE instruction.
Not secret skills...Just bouyancy and trim and propulsion, taken to the point that they are perfect for the diving situation, WHENEVER THEY NEED TO BE.
This probably sounds unlikley to you--unlikely that there could be any bouyancy or trim issues a 20 year diving veteran might not have mastered. When you see the skills first hand, and see them used in the natural setting they are intended for, this should change your attitude on the issue.
An obvious skill from Fundies is to learn how "not to" silt. While this came from cave diving, we have many uses for this in recreational diving. Getting close to the deck on a shipwreck, to be near a Goliath Grouper, or doing a minor shipwreck penetration ( with picture window size exits all along it) can mean being close to a very silty bottom--if you flutter kick near it, you will destroy the vis....if you need to be close to the bottom, maybe inches from it, there is a perfect way to do this without silting, but it is not a skill most long time divers would be proficient in. Maybe you need to be out of the current and close proximity to the bottom is critical....maybe you are looking for nudibranchs...their are many reasons.
Other skills are more about perfection in boyancy and trim as this will effect a safety or deco stop--where in a team scenario, you would want to maintain face to face connection of all team members the entire time....this means a perfection of each person in hitting and maintaining a set depth, and not having to carreen around all over in an attempt to hold depth ( thus losing eye to eye contact, and ability to watch a buddy's gas switch was performed with the right gas, etc) ... while this is more tech oriented, without the skill, future tech diving is hampered. If you never go to tech, it is still awesome to be able to hold an exact depth and flat trim as if you were being held in place.
Just a few of the skills are demonstrated here ...
YouTube - gue-reversekick1