So the way this works is the level has the max amount of 1-ups it can have, so when you collect one the blasters can now shoot one, triggering p-switches (In case you were wondering)
4:20 Damn, BeardBear is quite literally playing with death here, he really decided to get a life by collecting 100 coins AND getting the top of the flag despite the title
Get it is easy, if you collect a 1up you have some seconds before the launcher triggers the p-switch, so just collect the nearest 1up to the exit door, you'll have some seconds to reach the door and escape alive
As always, showing us what happens if you get a shroom. Safety first. Also this is more accurate Because doing shrooms would probably make you die if you did it in real life?
It's about limitations on certain types of items on-screen. Once one of the mushrooms is consumed, that allows another item, in this case a spike ball, to spawn and hit the p-switch causing the floor to disappear.