h2odivr222: Next to the ball/swing control are two white arrows. They usually appear across. If I touch them they will then appear up and down and freeze the green.
The green isn't freezing, the direction the dots are moving changes to show you the slope. One arrow gets you left to right, the other gets you front to back. That's the way it is on the PC and I'm assuming mobile is the same.
*edit-It's the same, just checked. :-)