Before considering simple chains, it’s important to understand strong links and weak links.
As shown below, a link where only two numbers are possible in a row, column or box, is called a strong link. In the diagram, it is between  and .
As shown below, a weak link is a link where there are more than three numbers that are possible within a row, column or box.  and  are in three possible cells, which creates a weak link.
A simple chain is when you focus on a number and look for a
cell - strong - cell - weak - cell - strong - cell
cell - strong - cell - weak - cell - strong - cell - weak - cell - strong - cell
pattern that begins and ends in a strong link. In the intersecting point between the beginning and end of the link, that number can’t be entered (it also works when weak links are strong in the pattern).
As an example, it will end up looking like the diagram above.
If a  goes in the first cell, F4, it obviously can’t be in F2.
If  goes in the first cell, F4, then  will go in the last cell, I2, so again,  can’t be entered in F2.
The -strong-[not 1]…. pattern will end up in [not 1]-strong--weak-[not 1]-strong-, and so a  will always come in the beginning or end of the chain.
This is the technique, but it is quite difficult to find the pattern, and unless you are really looking for it, it will be hard to find.