Keyboard navigation is toggled on and off with the "k" key.

All of what follows only applies if keyboard navigation is ON.

Regular (no shift/control/alt modifiers) arrow keys move selected slot up/down/left/right.

"SPACE" cycles through the visible items in the selected slot from top to bottom,
   wrapping around, selecting them one by one. (Shift-SPACE goes in reverse)
   Deselects currently selected item (if any) if the selected slot has no visible items.

"m" moves the selected item (if any) to the selected slot.
"d" marks the selected item (if any) done (moves it to the "Done" column).
"DEL" trashes the selected item (if any).

"n" creates a new item in the selected slot (only "self" right now).

"ENTER" edits the text of the currently focused item (if any).

"R" toggles expand/collapse state of the current row (Importance).
"C" toggles expand/collapse state of the current column (Urgency).

"A" expands everything (like clicking the "Maxi" icon).
"a" collapses everything (like clicking the "Mini" icon).
"t" collapses/expands the column and row you're in (like double-clicking).
"M" toggles Multiline mode.

"u" performs the "undo" function

"h" toggles keyboard help visibility.