that's not how that would look like. nor how it would work in the first place. you can't create a low pressure zone unless you've moved the air from that area into another area. saitama can't create a low pressure zone behind his entire body unless the direction of the force he's creating is behind him like say a back kick, but he's punching forward so in this case the direction of the force is in front of him. the low pressure zone can only exist in the path of his punch. meaning the low pressure zone can only exist in the space between where his fist was when he started his punch to however far he moved the air in front of him. yes his punch would've have created a low pressure area but not in the way you're implying. in order to destroy a mountain like that air moving at subsonic speeds won't suffice. and atmospheric pressure air can only rush into a vacuum at subsonic speeds because the only thing that would be driving it into the vacuum is atmospheric pressure which a mountain handles perfectly fine. so the shockwave couldn't have been from air rushing in. it had to have come from something with much higher pressure or energy. the shockwave must've come from saitama rapidly compressing the air in front of him and creating an extremely high pressure area and then the air exploding from the pressure once his punch ended. However, creating this high pressure concentration of air does create a trailing low pressure area behind it which air will rush into and this is probably what you're seeing. problem with what you're saying is that it can't happen behind him, it can only happen in front of him in this scenario. basically it's like saitama pushed a pocket of air and everything in that pocket of air's path was collected into this mass of air and left a vacuum behind it and air rushed in, which is what you're seeing at 0:21 and then rebounded slightly which is that last puff you saw. and if you're wondering where the force of the recoil went, it went to saitama, saitama tanked the recoil of his punch.