As with many free trial offers, you are asked to enter your credit or debit card details as part of the sign up process, payment will be taken from this card at the end of the trial for the agreed post-trial price. You have the option to cancel the subscription before the end of the trial period and if you do, no payment will be taken.
After downloading a magazine subscription trial, if you don't want to continue with the subscription after the end of the trial period, please ensure you cancel it at least 24 hours before the free trial period ends (normally 30 days or less), otherwise you will be charged the full subscription rate. Instructions on cancelling an auto-renewal can be found here.