It is basically the same principle as having paid by cheque so instead of being an instant payment it will take several days to clear before the funds are confirmed as paid & your item can be sent out to you by the seller.
Unfortunately Paypal provides very little information about E Cheques on their website & what information that is available is somewhat misleading & unhelpful.
Paypal state an E Cheque normally takes aprox 4 - 5 days to clear but it has been my regular experience that the normal timespan is 10 days & sometimes more for a buyers E Cheque to clear & for Paypal to confirm it is ok to send the buyers item.
Paypal normally sends an E-Cheque when the buyers bank details are unconfirmed or the Bank Card details are out of date, plus it is also well worth taking the time to fully register & confirm all details on Paypal such as confirmed address, bank account etc.
Should the buyer not want to wait 2 weeks to get their item & thus want to change the payment style from an E Cheque, then they should firstly check their card details held by Paypal are up to date or add another Credit Card to their account, then email the seller to cancel the E-Cheque and request to be reinvoiced for payment via eBay which should then go through as an Instant Payment.
I hope the brief info above helps and please let me know if you want any further advice re Paypal E Cheques & if you found the guide helpful please take a moment to vote Yes as helpfull at the bottom of the page.