This can be demonstrated with the following example:

Assume Reliance stock is selling at 1000 on the NSE and 1004 on the BSE. You may make an intraday arbitrage trade by buying at 1000 on the NSE and shorting at 1004 on the BSE. When the price difference is less than 4 (1004-1000), sell on the NSE and purchase back the shares on the BSE. If the difference between NSE and BSE prices is less than 4, you make a profit; otherwise, you lose.
This is an arbitrage trade since you are only concerned with the price differential between Reliance on NSE and BSE, not with whether Reliance moves up or down.

You can only make an arbitrage deal if you already have equities in your DEMAT account. So, if you had Reliance in your DEMAT, you could sell it at 1004 on the BSE and instantly buy it at 1000 on the NSE, lowering the cost of your holding. You won't be able to make any new intraday sell or purchase orders on multiple exchanges.