1) if a tank is on concrete, if you shoot at the ground in front/behind the rounds bounce up underneath and penetrate the under armor.thus disabling/killing the tank if you do not have bombs.
i would rank this myth right up there with russians intentionally ramming their T-34's into tigers and destroying the tiger.......
the armor on the belly of a tank is thinner than the top, but it was also built with the understanding that tanks were prone to running over many many types of potential dangers. the tank was given enough armor (panzer 10mm tiger 25mm) on the bottom to ensure it would likely survive most of the potential dangers it was likely to encounter.
now a .50 is rated to penetrate 22mm of armor plate at 100yards..(i think that is correct) now this is a direct fire, no ricochet into a flat plate of steel no angles. none of those circumstances are going to exist in the scenario laid out in the OP.
you have to first find a tank sitting on concrete or cobblestone, tar would be too soft, then get that close when firing, you have to be at the perfect angle to cause the bullet to bounce (how much energy the bullet loses impacting the ground and then bouncing is on i will leave to those that have phd's) at just the right angle to allow it to hit the plate flat (the bullet must remain intact and undeformed or these imperfections would further decrease the likelihood of any penetration), no angle (remember each degree of angle thickens the armor the bullet must pass through) then, only after all of these circumstances are met perfectly can it be said that you obtained penetration of the armor. then would come the final question.......after all that would the bullet have even enough energy left to penetrate the boots of the man standing inside?
as far as damaging the engine, well every photo i have seen shows the armor plate covering the tank belly running from front to back, no opening left to access the motor.
(oh and i use the american .50 cause its either the only round with enough energy to impact concrete and possibly bounce and maybe have some penetration potential left or the only round weak enough that it may bounce off of instead of penetrating the concrete 100% of the time)
now on armored vehicles, well i could believe that they are accessible through the bottom.....but the armor covering the entire vehicle is so thin to begin with why would you try to play a bank shot when you could just pound rounds through the top of the hull.
well those are my thoughts on the matter....