Plank
It does not time how long you lay there. It times how long you held.
An ordinary stopwatch measures the time during which you said you were holding. This one measures the time during which the posture actually held, which is almost always a smaller number, and that difference is the whole point.
What counts as holding
The phone in place
Someone in a forearm plank, facing the phone, which stands on the floor in line with their body. Both shoulders and both elbows are visible.
1200 × 900
Facing the phone: lying down, and resting on your arms.
Filmed from the front, your body must lie in line with the phone, not stand or sit in front of it, and your elbows must stay under your shoulders. Hips pushed very high, or dropped to the floor, leave the position too.
From the front, your feet are hidden behind your body. A back straight to within a few degrees, and resting on your feet rather than your knees, can only be checked from the side. That placement is still offered, and there the camera checks three angles, all three at once.
The body line
Shoulder, hip and ankle close to a straight line. Hips that sag, or that rise into a peak, both break it.
The knees stay straight
A knee that bends turns the plank into another exercise, and the clock stops rather than counting a position you are not in.
The arms stay under you
An upper arm too far from vertical means the weight has moved somewhere else. Three hundred milliseconds of valid posture are needed before the clock opens at all.
The rule that surprises
Stopping pauses the clock. Resuming loses nothing.
Hold for twenty seconds, lie down for ten, get back into position for twenty more: your score is forty seconds, minus the three hundred milliseconds it takes to confirm the posture when you resume. The time on the floor is never credited, and the time already held is never taken back.
The clock runs on the moment each image was captured, never on how fast your phone analyses it. When the posture breaks between two images, the time is split at the midpoint; when the camera sees nothing for more than half a second, that gap is not credited at all. A slow phone and a fast phone that saw the same plank give the same time.