How do you execute a certain sword attack?

Pretty straightforward. Press the attack button two times, pause for about 1 second, then start mashing it - you'll go into the 'million stab' combo that you described.

You can do another combo by pressing the attack button two times again, pausing for a shorter period of time (half a second), then pressing attack three more times to do two uppercut slashes followed by a downward one.

And finally, you can substitute moves, so you can substitute eg a High Time for the final stab or the downward slash of the previous two combos.