Sprite Clips

The first two require little to no effort, but the last one you need to time. You need to be in the position where you are two frames from not only touching the ground but also from hitting the P Switch. You have to press B on the frame you are two frames from the ground, since it won't register that you hit the POW and instead register a jump.
Showing my lua script because I'm kewl like that.

[@SpiceDawgCanimation]

That was mentioned in the Let's Glitch Super Mario World series.

[@Amaraticando]

Thanks for the info I. On the P-switch, sometimes I step on it and zero my x speed.

[@shadowdragon121TASer]

Something I might mention to you... this is actually possible on message and light boxes, but you must be at the upper half of their hitbox upon hitting their position. However, there's a way to manipulate your position to actually get to the lower half. This doesn't stay like that for long however, since yoshi is set to warp to the top of the hitbox when in it, so you must quickly get out. Something a bit similar: hitting a pow just at the bottom of a sprite block yoshiless warps you out.

[@shadowdragon121TASer]

You can also clip multiple sprites at a time if they are at the same position or nearly close to each other.

[@Amaraticando]

The same principle of lateral clipping. Mario and Yoshi aren't a rectangle, their compound hitbox has a hole,]

[@Canenari]

What the....:0

[@marioworldx]

WTF OωO