The direction doesn't actually matter since you're trading both sides. An example:
Exchange 1: 1000
Exchange 2: 900
You sell 1 of exchange 1 and buy 1 of exchange 2.
Scenario 1: Exchange 2 rises to meet exchange 1, exchange 1 stays stable. You make $100 as you bought exchange 2 at 900 and its price is now 1000. You lose nothing on exchange 1 since the price hasn't changed.
Scenario 2: Exchange 1 falls to meet exchange 2, exchange 2 stays stable. You make $100 as you sold exchange 1 at 1000 and its price is now 900. You lose nothing on exchange 2 since the price hasn't changed.
Scenario 3: Exchange 1 rises to 1100 and exchange 2 rises to meet it. You make $200 as you bought exchange 2 at 900 and its price is now 1100. You lose $100 as you sold exchange 1 at 1000 and its price is now 1100. The ultimate profit is $100.
The different exchanges are selling the exact same instrument. It's like having bank A offering $1.10 CAD for your USD, and bank B offering $1.20 CAD. If the spread is unusually large (eg, 9%) due to transient factors, they will definitely converge again in the future. There might be other risks involved, like counterparty risk, or fraud risk. But with a spread that large for an instrument as liquid as Bitcoin, there is no market risk.
Exchange 1: 1000 Exchange 2: 900
You sell 1 of exchange 1 and buy 1 of exchange 2.
Scenario 1: Exchange 2 rises to meet exchange 1, exchange 1 stays stable. You make $100 as you bought exchange 2 at 900 and its price is now 1000. You lose nothing on exchange 1 since the price hasn't changed.
Scenario 2: Exchange 1 falls to meet exchange 2, exchange 2 stays stable. You make $100 as you sold exchange 1 at 1000 and its price is now 900. You lose nothing on exchange 2 since the price hasn't changed.
Scenario 3: Exchange 1 rises to 1100 and exchange 2 rises to meet it. You make $200 as you bought exchange 2 at 900 and its price is now 1100. You lose $100 as you sold exchange 1 at 1000 and its price is now 1100. The ultimate profit is $100.
The important thing is that you trade both sides.