I'm going to assume you've never done any form of athletic activity at all. Never been shoved when running, never had someone tap tackle you while playing rugby, never moved at anything other than a moderate sedate pace and perhaps been gently nudged by a pizza box.
If you're outstretched and extending your leg, and get shoved in the back, then it has a high chance of causing you to be unable to push off your back leg to move onto whatever it is you're stretching for.
Now, i'm aware that's tough to consider if you don't have a clue about actually moving and running, but I can assure you that when you're at full pace or extending and lunging, small touches can make a significant difference, that you don't have a fucking clue what you're babbling on about.
And ohh look, the photo directly under this post highlights exactly how you've written utter rubbish...