Relative strength index

Whether recent close-to-close moves were mostly up or down, from -1 (all down) to +1 (all up).

Over the last N bars it adds up the close-to-close moves that went up and the ones that went down, then reports (up minus down) divided by (up plus down). +1 means every move in the window went up, -1 every move went down, and 0 an even split. It is the classic RSI ratio rescaled to -1 to +1, but it sums plain rolling totals instead of using Wilder's smoothing, so each bar drops out of the window sharply.

How to read it

The line sits in its own pane below the price, centred on zero and bounded by -1 and +1.
Above +0.5 the recent moves were mostly up; below -0.5 mostly down. On SPY one-minute bars with the default 14 bars, it sits beyond ±0.58 about one bar in ten.
A line pinned near +1 or -1 means one direction dominated the window. It does not by itself say price will reverse.

What it outputs

RSI: Net balance of recent close-to-close moves, from -1 (all down) to +1 (all up).

Settings

Window: How many bars the up and down totals are summed over. Shorter windows swing hard on single moves and hit +1 or -1 when a few bars agree; longer windows smooth out and leave the middle only on sustained runs.

Pitfalls

A window with no price change at all reads +1, not 0: the code returns +1 whenever the down total is zero, so a dead-flat stretch looks maximally bullish.
Unless the optional volume input is connected, each move counts the same whatever its volume.
The line is near zero whatever the price level (correlation with close about 0.04): it reads the direction of recent moves, not whether price is high or low.