When signing up for a free trial for BalletActive, a small pre-authorisation charge of $1.00 (around £0.73 in GBP) is placed on your card as a way to prevent fraudulent activity.
This charge will be credited back to your account within a few days as it is only meant to authenticate your credit card at registration. Please note that this hold will always be in USD, so your bank may convert the $1 hold to your local currency.