The virtual switch selects uplinks based on the virtual machine port IDs on the vSphere Standard Switch or vSphere Distributed Switch.
Route Based on Originating Virtual Port is the default load balancing method on the vSphere Standard Switch and vSphere Distributed Switch.
Each virtual machine running on an ESXi host has an associated virtual port ID on the virtual switch. To calculate an uplink for a virtual machine, the virtual switch uses the virtual machine port ID and the number of uplinks in the NIC team. After the virtual switch selects an uplink for a virtual machine, it always forwards traffic through the same uplink for this virtual machine as long as the machine runs on the same port. The virtual switch calculates uplinks for virtual machines only once, unless uplinks are added or removed from the NIC team.
The port ID of a virtual machine is fixed while the virtual machine runs on the same host. If you migrate, power off, or delete the virtual machine, its port ID on the virtual switch becomes free. The virtual switch stops sending traffic to this port, which reduces the overall traffic for its associated uplink. If a virtual machine is powered on or migrated, it might appear on a different port and use the uplink, which is associated with the new port.