A vertical stroke ..... In a character like 中 (zhōng - middle) it doesn't need any elaborations: a vertical stroke dividing a rectanglar right in the middle. Heisig uses the keyword walking stick (or cane, rod) for this component. In 丨+ 日 (rì - sun/day) we get the new character/component 旧 (jiù - past, worn, old), i.e you need a cane to take you through the old days.
In the first radicals/components (2-6) we will have to move below the "atom" level of the characters - components - and look at the "elementary particles" as individual brush strokes. This has little to do with the meaning, but the aesthetics of characters and how to improve handwriting. Yes, handwriting is an essential part of learning here.
The most important feature of vertical strokes is that they should be absolutely vertical. As you can see with horizontal strokes - like Radical/Component 1 - it's rather a rule than an exception that they are slanted upwards, but no angle whatsoever with strictly vertical strokes.
All Kanji/Hanzi as we see them today are created with a brush. To learn how to write them well you need to have a font - or examples - making this clear. - The nciku animations use a very similar font. - To use a font created for printed matters in small type is a very bad model: see the white on black characters below.
These strokes even have names: the one to the left is called "dropping dew" and the one to the right is called "suspended needle". The thin black lines show the approximate paths a brush would take when painting these strokes. When writing with a pen - nag, nag: preferably a fountain pen - it's not necessary to make the long trip back upwards in the dropping dew stroke, but the important point to distinguish between the clear tapering off in the needle stroke, where you gradually ease the pressure of the pen until the stroke fades out. In dropping dew you keep the same pressure and make a distinct stop and make a small upwards movement before releasing the pen. This is all about making the writing look ALIVE.