SSIA, or Short Stock Interest Arbitrage for short, is an extremely advanced arbitrage strategy only available to market makers that has made noise in the last few years. Click here for an in-depth white paper (PDF) on this activity. 

A few excerpts from the PDF to provide some more understandable context:

“A deep ITM put has no time value remaining and is priced at its floor value. Upon exercise, the put option holder receives the exercise price in cash. Each day the put option holder defers exercising the deep ITM put, he forgoes the interest income that can be earned on the cash proceeds, but retains an option to exercise the put on the following day. The difference between forgone interest income and the value of future exercise opportunities determines whether the put should be exercised early or not.”

“The failure of long put option holders to exercise early has given rise to a trading game. Since the interest income being forfeit by long put option holders is being earned by short put option holders, the game involves capturing short open interest. The game, dubbed “short stock interest arbitrage,” involves simultaneously buying and selling a large (relative to existing open interest), but equal, number of deep ITM puts and then immediately exercising the long puts. Since exercises are randomly assigned to open short positions, the arbitragers systematically capture the dominant share of the total short open interest and thereby earn the dominant share of the forfeit interest.”

Many traders have noticed vast amounts of deep ITM put activity coming across their feeds. We attempt to identify this activity and identify if with the emoji shown below. 

A tell-tale sign that you are witnessing SSIA taking place is to examine the contract's historical volume and open interest. If you don't see any significant changes taking place after sessions with high volume then you can confirm that the activity is from traders undertaking the SSIA strategy. Take the TSLA 450p Jan 2024 contract: we can see thousands of contracts being traded on a near daily basis with almost no change in OI. 

Similarly to the ITM call activity you see prior to a ticker's ex dividend date SSIA activity is arbitrage and NOT a directional trade.