Usually we talk about things travelling horizontally so there is no vertical vellosity and at the change of direction no horizontal vellosity. Similarly with the piston at the turning point it has no horizontal vellosity but as it is travelling in a circle it has a vertical vellosity.
I admit I must be wrong, I do dot understand the true answer.
And I will not be satisfied with that answer till I can understand for my self!!!
but thats my problem.
Sometimes you need to just take things on faith (ugh!) till later you learn more about a subject to look back and see we were right.
Thats the idea behind climbing one mountain of knowledge and not being able to see the whole picture till you climb another mountain of differant knowledge and at the peak of that mountain you see parts of the first mountain you could not see from that peak. Get the idea here?
Take the heads off an engine, crank it over by hand and watch the tops off the pistons as they reach and pass TDC. They stay there for a few degrees before and after true top dead centre. Try it with a degree wheel attached to the crank and you can measure it. Put some dial gauges on to find TrueTDC. Its done when blueprinting an engine, or tuning an engine for the perfect ignition timing.