The Chikou span is the "filter" of the Ichimoku System. Any signal given by the prices can only be validated after being filtered by the Chikou-Span.
On the graph below, Chikou is the black line (Chart created with FXCM Marketscope 2, click to enlarge)
Chikou is reflecting the closing prices, shifted 26 periods back in time.
I insist on the term 'closing'. Once the candlestick is closed, Chikou does not reflect the shadow of the candlestick but its actual closing price!
In my opinion, there are not 36 important things to remember about the Chikou Span! There are only two:
1) Everything (absolutely everything) stands in the way Chikou Span
2) Any signal given by the prices must imperatively and necessarily be validated by the Chikou-Span.
If you get a price signal, you have to make sure that at the same time that Chikou is free from any obstacle acting as a support or as a resistance. Everything is an obstacle for Chikou (the Japanese candlesticks, any Ichimoku line whatever, the trend lines, everything!) This rule of thumb for using Ichimoku must be applied on your main timeframe, as well as on any other timeframe that you may be using (see the tutorial about multiple timeframes).
BEWARE! This rule applies during a trend phase only. During a range phase, the rules are different (see the chapter about the Ichimoku Cloud).