Think small irimi which enters into the uke's space just enough that they cannot complete their movement without changing something.
Any time you "blend centers" with someone you have to displace their actual center of gravity. It doesn't take much ... and this causes an oscillation (or recovery cycle) which uke must do in order to try and get their posture and energy under their control again.
If you blend centers with someone without diplacing their center, it's called dancing for example.
The whole idea of aikido is to blend with and take the uke's energy and intent into your "keeping" and then make the decisions for the newly combined structure.
I think there must be "entering" to do this.