When does the build process update the checkout

I have configured a single build configuration that uses Git (github) as its VCS root.

Our repository is pretty large (a few GB in size) so i'd like to avoid full checkouts as much as it's possible.

It seems that the build process will checkout all the repo from time to time, even though it's not needed (it ran a few builds earlier).

Under what circumstances will TeamCity run a full checkout to update its git clone ? is this documented anywhere? (I did not run a "clean" build).