I think this only works if player lock has been set before the game in the profile.
During a game press B12, then select the B# for the player you want to lock. This is great because sometimes the player I want moves to a different position because of the rotation. For example a SG might spend some time at the PG position when subs happen.
This allows sticking with one player, regardless of position. Or, play as a different position for some time.
One thing it reveals is that whoever I am locked on, the opposing players corresponding to that position score the most. It's usually not the guy I'm guarding, but the other position in that area.
For example, if I'm locked onto PG one of the opposing back court guys (not the PG I'm guarding) score a lot. I'm guarding my guy, but maybe not helping as I should be.
If I'm the center, there are usually a lot more points scored by the opposing PF or even back court guys slashing in.
In general if I lock onto a big man the opposing team scores a lot from the paint, if I lock onto a back court guy they take more outside shots and score there.