It's a dance related to the song "My Boo" you move your arms in the same motion playing a keyboard. But you have to be located in a dark corner with a dirty look.
"Wow she's goooood I wish I could darkdirty keyboard that good"
when someone finds something funny and replies with random keyboard mashing rather than saying lmao or lol to show they're amused
mostly gays, preteens, or tumblr users use this tactic (or sometimes all three)
Gail: yo i fell down the stairs again
Sara: gGgHDGGDHShhhhdgdskskKDH
Gail: what the fuck was that
Sara: gay keyboard mashing
Gail: oh
Variation on "your lips to Gods ears" - may the (perhaps improbable) wish you just typed be received by the Divine, and then brought to fruition. Another way to say, "Boy, I sure hope so!"