When electric current passes through the coil in a magnetic field, the magnetic forces (F=BIL) produce an anti-clockwise torque which turns the DC motor.
When the coil crosses over to the other side, the magnetic forces produce a clockwise torque instead which would slow down the rotation. This is of course undesirable.
Usually, a commutator is used to reverse the current (and thus magnetic forces on either sides of the coil) during each cross over to keep the torque in the same direction.
What this video showed is a cheap alternative to the commutator: The coil’s insulation was sandpapered away only on one side. So when the coil flips over, it stops making electrical contact with the power supply. Without any current in the coil, there is no magnetic force. So the coil only experiences an anti-clockwise torque for half a cycle, and zero torque for the other half. Now we have a commutator-less DC motor that works by working by working only half the time.
Note: To be accurate, the connection is switched off completely only when the coil is horizontal. So the clockwise torque is not completely zero, but it is decreased enough for the design to work.