An mentioned into About Transaction, most of the Daobab transaction usage is automatic.
Every execute() usage start a dedicated transaction to do the job.
If you call execute() having opened transaction (OpenTransactionTarget), a new transaction will be not opened, but current one will be in use.
More about transaction propagation you may read here.
If you know, that there is already an opened transaction, just use executeNoTx(). This method uses current transaction without checking.