Been revving engines in my cars at traffic lights and in parking lots at meetups for decades without issue. Did that in Dad's cars back in the day too, and they were automatic.
So the idea is that the engine, without stress from the transmission being engaged, will blow up or be damaged because there's no mechanical stress placed on the engine from wheels turning? Never heard this reasoning before.
Edit: I do have to confess - I did break the motor mounts in Mom's LTD and Dad wasn't happy
FWIW, Here's a little related info
When the transmission is in neutral and the engine is “revved” without any load, the spinning engine internals will accelerate, gathering rotational and lateral forces at a faster rate than designed by the manufacturer.
Why will the engine wear? (not an exhaustive list):
Piston compression ring expansion
Rapidly revving an engine will heat up the piston rings much faster. Because they have a much smaller thermal mass than the cylinder liners they expand at different rates. If the compression rings expand too much, they will generate increased friction on the cylinder liners, causing the cylinder liners to wear out (reducing compression).
In the worst case scenario, the compression ring ends touch, pinch the cylinder liner and will most likely cause the piston to crack. Then you’ll have metal fragments flying around destroying the head and cylinder.
On a very cold engine or while lean (not enough fuel), it is easier to get a larger thermal differentiation between the cylinder liner and piston rings.
General, the cooling system is NOT
able to help, as piston failure happens within 4-10 milliseconds after a hotspot develops on a piston or compression ring.
Oil system starvation
More common in continued over revving - the cylinder head & block doesn’t drain the oil at the same rate that oil is pumped out of the sump, leaving the sump empty.
This can also be caused if the block uses the same pipes to drain the oil that is also used to ventilate the sump causing oil vaporisation. However the engine would most likely already be overheating for this to occur.
Some poorly designed oil pumps (and water pumps too) can aerate and are unable to pump oil when increased in rpm too rapidly.
Connecting rod (conrod) warping
Some conrods will stretch/bend during excessive rpm acceleration. Worst case scenario is it will put greater unbalanced forces on the crankshaft and bearings (higher compression ratio only needs 10 thou). If the engine has tight tolerances it will also bend valves.
The crankshaft or engine block isn’t designed to withstand such a sudden increase in internal force (similar to harmonic imbalances). It only takes a thou or two (0.0254 - 0.0508 millimetres) for the crankshaft to gouge a main bearing. This is common in performance & motorsport engines so they have increased strength built into the block by adding more ribs and webbing to the cast or billet."