Rulings Case Study 1: Battle Rules

Case: Demonic Dragon Berserker, Chatura vs Taboo Star-Vader, Rubidium

Okay, first you guys would need to have a copy of the Comprehensive Rules, you can get it here:
http://cf-vanguard.com/en/wp/wp-content/uploads/Cardfight-Vanguard-Comprehensive-Rules150619.pdf

Referring to page 11, Item 7.3. “Attack Step”, sub-item 7.3.1.3 and 7.3.1.3.2

“7.3.1.3 The turn player chooses one of his standing units on a vanguard or rear-guard circle in the front row (or if the vanguard is in a legion state, both units on a vanguard circle) as the unit to attack with for this battle, and rests it. If he or she cannot choose any units, then proceed to the close step immediately.”

“7.3.1.3.2 Only units that can attack at the point of 7.3.1.3 may be chosen be an attacking unit. Between when the attacking unit is chosen and the end of the battle, if the chosen unit becomes unable to attack, the process of battle still proceeds.”

At this point, the attacking player chooses a rear-guard Demonic Dragon Berserker, Chatura (Chatura) as the attacking unit for that battle at (7.3.1.3). Demonic Dragon Berserker, Chatura has the abilities:

“[CONT](RC): This unit cannot attack a rear-guard.

[CONT](RC: During your turn this unit gains POWER+3000 and "[AUTO](RC):(CB1)When this unit attack hits a Vanguard, you may pay the cost. If you do, draw a card and choose a card in your opponent's Drop Zone, bind it."

Continuous abilities does not require a check timing to activate, thus the ability is applied at all times. As such if the opponent only has rear-guards, Chatura could still attack but instead of actually battling, the battle straight away ends (as denoted by the orange text above).

The opponent, having a Vanguard (which is mandatory, otherwise he loses the game) provides one valid target for Chatura, allowing it attack, so the battle moves from item 7.3.1.3 to the subsequent steps. Note that Chatura is now an attacking unit and is governed by item 7.3.1.3.2.

Since Chatura is attacking a valid target, the battle moves on to the next step; page 12, Item 7.4 “Guard Step”. 

Nothing special happens here, just the usual Guardian Call and effect resolution. Now, the opposing player plays a Taboo Star-Vader, Rubidium, using its effect to redirect the Chatura's attack target to a 'Reverse' unit in his rear-guard. This counts as Chatura attacking the rear-guard. 

Now, recall that Chatura has a [CONT] effect that prevents it from attacking a rear-guard. There is a check timing between the Guard Step and the Damage Step (it is a rear-guard attack so we skipped the Drive Step), but being a Continuous ability, the ability kicks in and prevents Chatura from attacking. However, recall that last part of item 7.3.1.3.2, the red text above;

"7.3.1.3.2 [...] Between when the attacking unit is chosen and the end of the battle, if the chosen unit becomes unable to attack, the process of battle still proceeds.”

Thanks to this handy rule, the attack still goes through as the restriction is (re)-placed AFTER Chatura had been declared as an attacking unit with a valid target. So the key rule of units with restrictions on their attack targets is that if the attack target is changed after attack declaration, the attack will continue (targeting the new target) even if the change in target would cause the restriction on the attacking unit to be activated.

Conclusion: In a match where Rubidium is used to redirect a Chatura's attack to a rear-guard 'Reverse' unit,the attack will continue targeting the new target. Rubidium does NOT nullify the attack.

A/N: Chatura is still MVP (no Touken).