I can only let my car run in the parking that long if I have an oil well in my backyard!.

I presume you already know some parts of an engine such as crankshaft, camshaft, injector, etc. To recap Mm, the crank converts the up/down movement if pistons into circular motion, and the cam controls the opening & closing of inlet valves (air and fuel) while the fuel injector does exactly that.

Now, in order for these parts to function in unison, they are interconnected via a timing belt, and hence what you might have heard your mechanic refer to as 'timing' especially hizo pande za ngirogoni. What this means is that the up/down movement of pistons closely regulated in tandem with opening/closing of valves so that the valves only open when the piston is at the maximum of the downstroke to inlet the air/fuel mix prior to ignition after which the exhaust valve will also need to open to let out the exhaust fumes, and on and on. You'll agree with me that these complex operations do indeed require very accurate timing.

Obviously, like all other car parts, the timing belt has a lifespan. After all, it being part rubber does indeed lose it's elasticity and indeed if you look closely at a well worn one, you can see some minor cracks along its outer surface. Now, imagine with me a situation where, in the midst of all this up/down piston movement and open/close valve operation, the timing belt suddenly snaps. What is likely to happen?

Bearing in mind the speed at which the engine was moving, several things may happen. One, the piston may come up and find the valve(s) still open. Once the valves hit the piston surface ;which they ordinarily shouldn't, you end up damaging both the valves and pistons, all of which will require replacement. Secondly, it is possible that when the belt snaps, the cam also breaks into small pieces (this happened to me ) and in the process the valves are again affected along with valve seats, seals and a myriad of those other small 'thingies' associated with it. Worst case scenario, the pistons might hit the cylinder head so hard that they cause it to warp/nick (not common) or cause it to crack (more common) and obviously necessitating machining if you're lucky or total replacement. This happened to my pal. On a polite drive from church, gari ikajizima tu yenyewe and refused to start. On inspection at garage, 'twas found the timing belt had snapped and taken out the cylinder head and it's components along with it requiring replacement. Total bill = 100k. Total cost of belt = 4.5k at the time.

And that, @impunity, is my layman's way of putting it. Hope I've made things a bit clearer. Next time you're at the garage and you find them rebuilding an engine (kufunga engine as Kariúki in ngirogoni calls it), get close and observe. Ask some queries on what was wrong with it for instance, what caused it and how to prevent the same from happening. You'll come out better informed and as always, forewarned is forearmed.