It's always interesting to be a pedestrian, standing at traffic lights waiting for the lights to change so you can cross. Watch any group of people on the opposite side as the 'walk now' sign lights up, and almost as a unit, all will lean to one side and then take a step forward.
This of course is because they are standing relaxed whilst waiting. When they actually want to start moving, they have to engage their muscles, shift weight on to one leg so they can move the other, and only then, start walking across the street. Even if you start with your weight already shifted to one side, if you relax, or let yourself 'sit on your bones', you still have to reengage the muscles and tendons before you can start to move.
The transition from still, to moving, requires a shift, mental and physical ... unless you wait in 'stand by' mode, like a runner waiting for the start gun, with soft tissue engaged and ready to go.
My Bagua teacher calls this 'stand by' mode, and likens it to being like a monkey that has seen a piece of fruit that it wants, and is waiting with full intent for that moment when they can grab it from the unsuspecting.
It's a rather interesting mode to be still in, and one that is practiced in many martial arts in standing postures, or in forms that hold moments of stillness within them. But it's hard to maintain in a relaxed manner .... hence the need to practice it. Internal arts call it 'Motion in Stillness', the partner of 'Stillness in Motion'. The transition between the two being a crucial place to work.
The other way to avoid this gap, this extra step to reengage the muscles etc for movement, is to always keep moving very slowly.
This came up in Toyama Ryu class the other morning where we were practicing random entries and reactions. If you think of the drill as a reaction to a random entry and set yourself up to wait for the opponent to commit, you are in effect relaxing, or at least disengaging a response from your legs until you see what's coming ... This is ridiculous and rarely works - there is not enough time to react, the OODA loop takes too much time.
To do the drill correctly, you are really never waiting per se but are constantly improving position and setting up actions, slowly, subtly, forcing the opponent's hand, and keeping your own mind and body engaged in the moment. You stay in motion, moving into your opponent, or changing line so there is no need to orient or decide, just observe and act.
It's a subtle thing though, this motion in stillness, Or this ability to maintain constant, slow and smooth movement, it's like a predator, stalking. You don't want to startle you opponent at the wrong moment, you want to entice them in, or remain unsuspecting until the moment you pounce.