In investing, it's sometimes important to look at the relative performance (or relative strength) of two securities. This is sometimes called pairs trading. For example, you may want to look at the performance of AAPL vs. QQQ, or FB vs. GOOGL. You can quickly look at the chart of relative performance on Koyfin.
One way is to use the AAA:BBB format in the command bar (To activate the command bar, type / on your keyboard). For example, you can type AAPL:QQQ in the command bar, and then select the G chart. This would pull up the graph of AAPL / QQQ.
You can also create a relative performance series in G chart. First, load a security into G chart. Then add a new data series called relative strength.
This will add another series to the chart with the relative strength. You can change the secondary ticker by clicking on the options of the series.
Relative strength (A/B) is the ratio of two prices and approximates the performance of Long $1 of A vs. Short $1 of B with daily rebalancing to make the trade $ neutral ($1 long, $1 short).
Relative spread (%A - %B) is the performance spread of two assets. This assumes an investors is long $1 of A vs. short $1 of B at the start of the chart and doesn't rebalance the trade.