The begin transaction phrase affects the behavior of short transactions.
If the begin transaction is issued prior to the end of the request, it triggers a commit of all previous statements in the request and temporarily sets the transaction mode to long.
If the begin transaction is not issued prior to the end of a request, the request marks the end of the transaction, and the transaction is committed.
You can issue a commit or rollback to end the temporary long mode. Doing this causes the transaction mode to revert to its status before the begin transaction was issued.
If the batch is in temporary long mode and ends (exits) without a commit or rollback, the SQL statements are rolled back, and the transaction mode reverts to its original status.