https://en.wikipedia.org/wiki/USB_hub
> During normal transmission, hubs are essentially transparent: data received from
its upstream port is broadcast to all devices attached to its downstream ports
(pictorially described in the USB 2.0 specification in Figure 11–2, Hub Signalling
Connectivity). Data received from a downstream port is generally forwarded to the
upstream port only. This way, what is sent by the host is received by all hubs and
devices, and what is sent by a device is received by the host but not by the other
devices (an exception is resume signalling). Downstream routing has been changed
in USB 3.0 with the addition of Point to Point routing: A route string sent in the
packet header allows a USB 3.0 host to only send a downstream packet to a single
destination port, decreasing congestion and power consumption.[12]
> Hubs are not transparent when dealing with changes in the status of downstream
ports, such as insertion or removal of devices. In particular, if a downstream
port of a hub changes status, this change is dealt with in an interaction between
the host and this hub; with any hubs between the host and "changed hub" acting as
transparent.