His explanation of relativistic timedilation, which causes fast-moving clocks to appear to be running slowly when viewed by a stationary observer, is exemplary.
Those variables are then plugged into the Lorentz Transformation equations, which are used to determine the total amount of timedilation that occurs during your run.
We have been saying since 2005 in our paper at physicsnext.org that, while different observes will measure different times in many cases, Einstein time-dilation equations are wrong, and we give the correct equations.