The BXBT Index is the average contract value taken at the rates of 3 fiat exchanges: now it is in equal proportions at 33.3% of Bitstamp Coinbase and Kraken, however, in the case of strong instability of one of the exchanges, the composition can be changed.
Here is how the index curve looks on the chart, it can be seen that the index periodically deviates and is above or below the price level of the Bitmex glasses
An index exists to minimize the likelihood of course manipulation directly inside Bitmex. It turns out that it somewhat smoothes out the extremes.
If there is a very long sharp shadow of a candle on Bitmex that touches your level of liquidation, then liquidation will NOT occur. Liquidation will occur if the same sharp drop in prices occurs on three other exchanges. And at the same time! And since there is no huge leverage on fiat exchanges, the likelihood of an extremely long shadow is several times less. Such a built-in system at times reduces the likelihood that your position will be liquidated due to the extremely sharp price movement directly to Bitmex.
Therefore, do not surprise if orders in the glass are triggered below your Margin call level (provided that you have a long position), but the position still does not close — this will mean that the average rate of the three exchanges was higher than your liquidation level.
The historical value of the BXBT index can be tracked at https://www.bitmex.com/app/index/.BXBT. Changes happen every minute.