Declined sharply, 5 letters
Did a full gainer, 5 letters
Did a gainer, 5 letters
Did a jackknife, 5 letters
Dropped sharply, 5 letters
Executed a gainer, 5 letters
Fell precipitately, 5 letters
Headed for the bottom, 5 letters
Jackknifed, 5 letters
Jackknifed, e.g., 5 letters
Jumped in, 5 letters
Left the board, maybe, 5 letters
Plummeted, 5 letters
Plunged, 5 letters
Plunged downward, 5 letters
Sought pearls, maybe, 5 letters
Tried a jackknife, 5 letters
Went head-first, 5 letters
Went off the deep end, 5 letters
Went scubaing, 5 letters
Went under, 5 letters
Began eagerly, as a project, 9 letters
Began with gusto, 9 letters