Digi-Xcros is a new mechanics (new rule) that is introduced from BT10 Cross Encounter booster set. There are many new effects of skills that this post is going to make it clear and easy to understand.
First of all, from the FunDigica video, we know some basic info of Digi-Xros:
How to trigger DigiXros
Let's take Shoutmon X4 as an example:
As the info in Shoutmon X4 card, trigger DigiXros by playing this card from hand and reduce the play cost of this card by 2 for each card placed. Cards that can be used for ShoutmonX4 DigiXros includes: Shoutmon, Ballistamon, Dorulumon and Starmons.
If you happen to have Ballistamon, Dorulumon and Shoutmon in hand or in the battle area or under your tamers to become materials for Shoutmon X4, the order of placing under ShoutmonX4 would be as the picture above (Point 5. Materials are placed from top to bottom in the sequence of left to right of the DigiXros requirements Digimon (like Jogress)).
So, by placing 3 Digimon under ShoutmonX4 (let's say you don't have a Starmons to use), now you only have to pay 3 costs (after reduce play-cost of Digicros). (But if opponent has a Digimon that blocks you from reducing play cost, then you have to pay 9 cost to play ShoutmonX4).
[On Play] effect of ShoutmonX4 allows you to [Draw 2], and with the Shoutmon, Ballistamon and Dorulumon under its digivolution source, ShoutmonX4 gains "Rush", "Piercing" and "all opponent's security Digimon gets -2000 DP". (So ShoutmonX4 can become the final hit to win the game).
And now, let's assume that you have a Kudou Taiki (Mikey Kudou) in the battle.
If ShoutmonX4 is deleted, either after the attack or in opponent's turn, the "Material Save 2" is triggered. You can choose 2 Digimon from ShoutmonX4's digivolution source to put under 1 of your tamer, for example: I would pick Shoutmon for "Rush" and Ballistamon for "Piercing" to be saved-material, place them under Mikey Kudo tamer.
For Mickey Kudo [Your Turn] effect, "when playing 1 of your Digimon with DIgiXcros requirements, by suspending this Tamer, you can also place cards from under your tamers into digivolution source by DigiXcros".
The question is: If I use the Digimon under Mickey Kudo to place under a DigiXcros Digimon, can I reduce play-cost of that DigiXcros Digimon by 2 for each card I placed from the Tamer.
The answer is : yes.
The reduce-play-cost is applied for all those Digimon that listed in DigiXcros requirement, no matter they are taken from the battle or under a tamer, as long as there is no duplication. So ShoutmonX4 can be played by paying 1-cost minimum.
In the case if ShoutmonX4 is not deleted after the attack, you can choose to trigger its "end of attack" effect if you have another ShoutmonX4 in hand. It allows you to place all its digivolution cards under 1 of your tamer in any order to unsuspend 1 of your tamer, and if you do, delete the ShoutmonX4.
This step helps to preserve all the Digixcros materials to do another DigiXcros for another ShoutmonX4. As long as you have enough memory, the new ShoutmonX4 can attack opponent's Digimon or player with "Rush" inherited from Shoutmon.