Calculation:

BULLS = HIGH - EMA
BEARS = LOW - EMA

Where:

BULLS - force of bulls;
BEARS - force of bears;
HIGH - maximum price of current bar;
LOW - minimum price of current bar;
EMA - exponential moving average.

Source Code "Bears Power":

function init()
{
   Bears.createParameter("Period", 14);

   Bears.createBuffer("Bears");
   Bears.setBufferColor("Bears", 'red');
   Bears.setBufferDrawStyle("Bears", DrawStyle.HISTOGRAM);

   Bears.setAutoChangeMaxMin("Bears");

   Bears.addLevel(0, "white");
}

function start()
{
   Bears.label = "Bears(" + Bears.parameter("Period") + ")";

   var num = Shared.numberOfQuotes();

   var bufSmaClose =
     Shared.ema(Bears.parameter("Period"), Shared.close());

   var bufBears = new Array(num);

   for (var i = 0; i < num; ++i) {
     var fValue = Shared.low(i) - bufSmaClose[i];
     bufBears[i] = fValue;
   }
   Bears.setBufferData("Bears", bufBears);
}