PrivateSend is a term used to describe both the mixing technology of Dash and the private sending of dash. The main Dash wallet includes this functionality as an option.
If you want to send a payment privately or erase all previous history, you need to mix your coins prior to sending. First go to your wallet options and select the amount of coins and how many rounds you want to mix. 2 rounds is minimum and 8 rounds is still unbreakable. You select Mix at the lower left of the wallet. It will deduct a 0.01 Dash fee to start mixing. The mixing process can take several hours to days depending on rounds and network traffic.
With your coins mixed you can still send a normal transaction, but it can be traced back to your wallet with the change address. To get around this there is a private send box that can be selected. This rounds up your payment to the lowest denomination (currently .1 Dash). Then when the payment is sent only a set of common denominations are sent 10s, 1s, .1s. It is not possible to determine which address the funds originally came from...................