First off, the thing to keep in mind is that most cross-ups are actually very easy to parry for an experienced player. The idea is that an experienced player won’t just tap backward to try to parry a move where they expect a cross-up because they understand that the other player is aiming for an ambiguous cross-up rather than something that can be easily detected, as if they didn’t, then crossing up in the sense of anything useful would be pointless. Keeping this in mind, most players with just tap forward when they suspect the move will hit, regardless of whether they can tell which side it will hit on or not.
(The following pertains to Ken, but can be re-worked for any character, with height of character vs. what cross-up options the opponent has being the two main factors.)
The reason is as follows:
If the cross-up is truly ambiguous, then there are four options an opponent has: hold forward, tap forward, hold back, or tap back. Since they don’t know what you are doing, we can eliminate the options presented to the best response.
Hold Forward: This will only either block the cross-up if it hits on the opposite side, or if they press and hold forward right when you do the move it will parry the first hit if you don’t cross to the other side. Thus, this will lose to multi-hit moves forward unless they can react quick enough to change to block/parry afterwards upon seeing it is multi-hit.
Tap Forward: This will either block a cross-up assuming they do it right when they expect the hit to connect, or it will parry one hit forward unless a multi-hit move is done again, which they will once again have to react and do something quick enough afterwards.
Hold Backward: This will either block any move done forward, or parry the first move backward if timed right when the move hits and then held.
Tap Backward: This will either block the first move forward in which they will have to react quick enough to continue blocking or red-parry the next move, or it will parry the first-hit from the back cross-up move, in which they will have to react quick enough if it can somehow go multi-hit.
Now, there are two main things to consider when choosing the best response: how many times in each option you will have to change you stance on a multi-hit moves (reaction), and how many of the four types of moves each one handles (one-hit non-cross-up, multi-hit non-cross-up, one-hit cross-up, and multi-hit cross-up).
Hold Forward: The worst option, as the only advantage would be blocking multi-hit cross-ups which hardly any moves do if you are an average-size character like Ken, and you are already committing yourself to parrying once forward, then taking the rest of the multi-hit move–which then forces you to react if forward.
Tap Forward: The best option as it blocks a cross-up hit, and since multi-hit cross-up moves rarely hit ‘multiple times’ from the back, you don’t need to worry after the first hit is blocked. Second, you are already in the mindset of reacting after the hit is parried forward/blocked backward (cross-up), thus the transition is already anticipated making it easier than committing to Holding Forward and also allowing more frame advantage from the parry to do a move.
Hold Backward: The Third best option as it blocks any multi-hit forward move, and parries the first hit of a cross-up move; however, once again, since rarely any move can multi-hit cross-up on Ken, dedicating to a Hold is pointless and causes one to unnecessarily react when the move is found to be non-multi-hit. To show how bad this can be, imagine if they only do a single-hit cross-up and you parry, immediately afterwards, if you hold back, you are walking towards them enabling easier supers from parries and right into throw range. If anything is learned here, it should be not to Hold any direction when anticipating an ambiguous cross-up.
Tap Backward: The second best option, since it will block the first hit forward, parry the first hit backward (since multi-hits rarely hit Ken, you don’t have to worry about subsequent hits), and puts you into a reacting pose automatically, meaning, once you block the first forward move it should be easier to continue blocking, and once you parry the cross-up move, you can easier react with an uppercut, or whatever.
The only reason the Tap Forward is slightly better than the Tap Backward is when comparing blocking the first forward hit of a multi-hit move to parrying the first hit (i.e. when using Tap Forward) you are given more time (i.e. frame advantage) to react to either another parry, or to a block. It is much harder to tap back once, block the first hit, see there is another hit, and tap and then hold back for the remaining hits (or red-parry the next hit).
Now, the next part is how to abuse this. Most experienced players understand this, thus the only hard part for them would be timing to Tap at the appropriate time for your move to hit. Thus, the obvious choice would be to try to make this timing as difficult as possible, and lucky for Ken, you can do this with his EX Tatsumaki. Using this, I will give an example of how to use this efficiently midscreen.
First off, keep in mind that Ken has different cross up properties midscreen depending on whether he is 1st or 2nd player (1P or 2P). An example (and I credit this to Mopreme’s excellent 3S journal) is when on the 1P side, after connecting with Ken’s super Jinrai, Ken is placed in a curious position, where upon super jumping exactly when he lands will creating a situation that looks very ambiguous but will cross-up the opponent. However, if one hesitates for even a moment and then super jumps, it will still look ambiguous, but not cross-up the opponent. This does not work on the 2P side. The usefulness here is that in order to hit the opponent on cross-up with the tricky EX Tatsumaki I was talking about you will have to wait (as the EX will curve and speed up the arc and thus missing them right as they are getting up…but then again, you can use this to slightly trick your opponent as well into committing to a Tap and then landing with a quick ground link->super or kara-throw), but if you wait, you will not cross-up. Thus, in order to get around this, tap forward slightly right when you land and then super jump, allowing more time to pass when they get up and moving you just enough forward so that you still cross-up the opponent. And with these two options (pause then super-jump, vs. forward then super-jump), you can create a perfect situation to disrupt their timing by using an EX Tatsumaki right when the ambiguousness is created–meaning the opponent will be even more confused as their time to choose how to parry and when it sped up and increasingly difficult.
There are other good examples, but this is one of the best. Just experiment a bit and try it on different characters, as their different heights can create some interesting options as well : )
I hope this helps in determining Kens cross-up block options, and just remember to be aware to what multi-hit moves you opponent has in these ambiguous settings as well as Ken’s.