when using numpad keys Num_1 for upward movement, Num_0 for downward movement. usig right shift for something else (i have it for break in flight mode and fast move in build mode)
holding "Num_1 or 0" then holding "right shift" then releasing "Num_1 or 0" after that releasing "right shift" the movement continues now until "Num_1 or 0" are pressed again.
happens in Flight mode, Astronaut Mode and Build Mode