Everything is reactive and nothing is guaranteed to work. There, I said it.
Just to clarify - You are always reacting, even if you 'go first'. What I mean is, you might do a preemptive strike because you sensed something coming, but you still reacted to the behavior of your opponent.
Martial arts teaches us all kinds of techniques - If they do this, you do that, whether it's something specific to do with stepping off line and executing some series of moves, or more vague like 'if they close' step in, keep hitting until you overcome them'.
But would you still do it if you knew it might not work?
How about if it only had a 50/50 chance of working?
Having only one single option in hand with no room for adjusting to changing circumstances means you are always going to be behind the timing. Convince yourself that your first entry is going to work no matter what means that your mind will be stuck in 'ACT' phase of the OODA loop with no time to OBSERVE or ORIENT to a change in circumstance.
Remember, even if you think you are acting preemptively, sometimes your opponent is just waiting for you to come into range so they can connect and control your movement. Nothing is guaranteed.
Most martial systems understand this and have a part of their training specifically dealing with change, usually from contact - Push Hands, Chi Sao, San Shou, or what Sonny called Second Flow.
In Yizong Bagua, this is where Line 3 comes in - That moment where contact is made but your initial idea needs to be altered.
The initial entry may be a good one, so you cannot act like it is definitely going to fail, but you still need some more 'ideas' some options, that you can potentially use if it does.
In Bagua we talk about 'half' - I do half, my opponent does half. We also talk 'testing', 'saying hello', about 'keeping a good situation'. In Eskrima we talk about 'not running out of angle'.
What we are doing is forcing a reaction from our opponent that we can use to our own benefit.
This means there has to be both a precision of relative position between you and the opponent so you have the structure and power to do the next thing you want at any moment, but also a fluidity to smoothly segue into that next thing, whatever it may be. All whilst preventing yourself from taking damage.
This requires good alignment (in yourself and between you and your opponent), an understanding of the strong and weak angles of the body, and the sensitivity to know where one option to continue is better than another.
Last but not least, an understanding of the concept of 'half', where 'half' is.