Hi, Jean
As already stated by Norman and Eileen, Mega voices are not intended for playing, only for styles and MIDI files. The issue is that these voices pack different paying techniques into a single voice, by using how "hard" you press the keys to trigger these differences.
How "hard" you play is called "velocity" in MIDI terms, and takes values from 1 (extremely soft, almost impossible to accomplish) to 127 (extremely hard, also almost impossible!). The full information on what is packed in each voice can be found in the Data List manual for the keyboard, which can be downloaded from Yamaha. An example to clarify:
Mega Voice: CleanGuitar (Mega voice)
Velocity Effect
1 - 20 Open soft
21 - 40 Open hard
41 - 60 Slap
61 - 75 Dead
76 - 90 Mute
91 - 105 Hammer
106 - 120 Slide
121 - 127 Pick Harmonics
(for each sound you still can control velocity, but in a smaller range).
It is impossible for a human player to play consistently with the desired "strength" to control the different nuances, but using a MIDI editor (or the Style Editor) this is not difficult at all. That's where these voices shine. If you are a DAW user, definitely consider using these voices to create more realistic MIDI files (but beware: the resulting files will only work fine with Yamaha keyboards having the selected voices, and will sound awful otherwise!).
It is interesting to note that the actual samples used are shared with other non-Mega voices. Many of the Super-Articulation voices use the playing style (legato/staccato/accents) plus the keyboard controls to select the actual sound used.
Regards!
Andres
