The screen photo helps explain what's going on:
Inside the red circle is a downward-pointing arrow. It marks the time (on the horizontal scale) where the 'scope triggers. The horizontal position of this arrow can be slid left or right with the front-panel horizontal position knob.
At the top of the screen appears text M Pos: 0.000s This tells you that the trigger point is exactly at the centre of the screen (equidistant from the display left edge and right edge). This display is very useful for measuring time spans: its number will change as the trigger control moves the downward-pointing arrow left or right. It displays a signed number of seconds, milliseconds, microseconds, nanoseconds between cursor and centre-of-screen. When the downward pointing arrow is at screen centre, its display is 0.000s.
The black T along with Trig'd tell you that the 'scope circuitry has successfully found a point on the waveform (of the proper slope) on which to trigger.
At the right edge of the screen is a left-pointing arrow. This arrow slides up and down, adjusted by the trigger level control. It controls the voltage level of the trigger point (as marked on the photo by the orange horizontal line). As this trigger point is moved up, the sine wave would shift left a little on the display. If the trigger voltage moves above its topmost peak, the 'scope becomes untriggered, and the nice stable display goes all squirrelly. The black T would also change, indicating that the display is untriggered.
Most digital 'scopes default to trigger at the centre of the screen. Unlike old analog 'scopes that must default to trigger at the screen's left edge.
Coordinating all these knobs, displays takes some time to master. Much thought has gone into making scope measurements versatile yet simple - two conflicting issues.
