After the initial moment of contact or engagement, a moment arises where the nage pauses to assess the situation.  I say momentary because this pause depends heavily on the practitioner's level of training.  I quantify the pause as a moment because only you can define how long a moment is because it depends on one's level of training. Generally speaking... Beginners take a more physical pause, up to a second, where they assess if the have either broken their opponents balance or if they are in the proper position.  With this information, they proceed accordingly.

For intermediate practitioners,  they work to decrease the amount of time they pause to assess the situation but there might still be a noticeable physical pause.

For advanced practitioners, the pause becomes more mental and may not even be externally apparent to the uninformed onlooker.

What would the pause look like for a technique like sumi otoshi after the initial attack and irimi movement... The beginner pauses to see if their irimi movement was deep enough and if their opponents balance is broken enough to throw them.  If not they will have to inch in deeper to make the throw work.  The pause could be up to one second.

The intermediate level person can physically pause too, but their pause should be ever so slightly physical and inching toward the mental.  They try and cut down the amount of time they pause, but they still pause.  I would say that you should be able to barely make out the physical pause.

The expert's pause is purely mental and almost looks like they don't even physically pause between the initial irimi and the second step through.

This idea of a pause is heavily associated with the idea of zanshin which many associate with the ending of the technique in Aikido.  Zanshin really means a state of total awareness where the mind doesn't rest or stop at any one point.  The reason I say that the pause and zanshin are tied together is that the pause is zanshin at work.  For a beginner, it is easy to loose focus so the duration of the zanshin only extends to after the initial irimi or tenshin movement after the onset of the attack.  The more experienced person tries to maintain zanshin throughout the movement.  After one is able to maintain zanshin throughout the technique and demonstrate it at the end of the technique, one tries to extend the zanshin longer.  They try and make it last more than one technique, then throughout the whole class, then for a few hours, then the whole day and eventually into every moment of one's entire life.

Zanshin is a state of total awareness that begins with a pause.