He has shown that his dogs have no teeth...So no problem there...

Here are two things which can cause serious consequences using air to pressurize a cylinder in which the piston is NOT at the bottom of the stroke...

One... the timing pointer can be damaged .... or the wrong one be on the engine... which causes you to think your piston is exactly at TDC when it is not... I have seen both those things ... not in connection with pressurizing the bore fortunately ... but you can figure out what would happen... it only takes a few degrees off TDC to cause a problem.

Two... some cars have the crank timing mark on the balancer which is rubber mounted to the front of the crank... I have seen old ones which had rotated as much as 90 degrees.

While that much difference should show up in the position of the valves... less would not... and the amount of pressure ( 4 x pi x 30 ( plus 1/2 ft at 90 degrees ) ( or times 100 when doing a leakdown test)... can thrown a tool attached to the crank bolt, turn the engine backwards, catch a finger in a belt, hit your hand with the fan blade.... just too many things which are easy to avoid completely with the rope.

And that is not even mentioning that if it goes far enough to activate the valve mechanism on the exhaust it will take away the pressure and let the intake drop into the bore...

I have had some close calls in my 40 years of DIY car work.... but I still have all my fingers and eye sight... we do not know for sure about others who post with lots of bravado when they are not going to get hurt if something unusual happens. I feel we as a group have an obligation to go with the safest advice we can give since we have so many levels of experience potentially reading the information.