Does Splitit hold the full amount on my credit card?

Yes.

In addition to your monthly instalment payments, the full outstanding amount is authorized (held) on your card to guarantee future payments. Authorizations are usually renewed every 17-21 days until your plan is paid off. Authorizations get smaller with each instalment paid. The previous authorization is released once a new one is obtained. Authorizations show as 'pending' or 'reserved' on your account. If your plan is kept on track, these authorizations are not charged. 

You can find out more about payments and authorizations here.