10.X1 lockscreen modes explained

A: In any lockscreen mode, pressing the power button locks/unlocks the screen.  While the screen is locked:
In Lockscreen mode 1: only volume buttons respond; press briefly to change volume, hold to skip a track

In Lockscreen mode 2: in addition to the above, the OK button in the center responds; press it briefly to play / pause / unpause playback; hold it to unlock the screen.

In Lockscreen mode 3: in addition to the above, the previous / next buttons at the bottom respond.  Press briefly to skip to the previous / next tracks; no response on holding down these buttons in lockscreen mode.