Open market buybacks happen spontaneously. In Infosys' case the cut off price is INR800, and the total size of INR8,260 Crore.

They will keep on buying equity as long as the share price is below INR800 and the corpus of INR8,260 Crore hasn't been used up. If either of these happen they will stop.

Given this, there is a natural liquidity in the stock which will bolster the price towards INR800 after which the liquidity dries up and it will start going down again. Now, how close it will come to INR800 is where the whole thing about daily traded volume, mean quarter sigma etc. comes in.

You can't directly participate in the open market buyback unlike in tender offer.

Like I said should, but to be more technical, it does happen, particularly after the Ex-date, which many times is not same as the disbursal date. This is where the hard part about arbitrage comes in. When does the stock adjust to the event it could adjust the next second, it could take well over a month, depending of the market liquidity of the stock.

General rule of thumb - more liquid the stock is, the quicker it will adjust to event and harder it will be to spot the adjustment.