Can someone summarize that part of IRS Pub 721 that allows a rollover of a US Gov annuity to an IRA (TSP) tax free? TT allow this option but does not explain why.

I read an article that explained why I can contribute pre-tax $ from my pension (annuity) but cannot find it anymore. The IRS Pub is very confusing about what is and isn't allowed. I was able to contribute a small portion of my payment to my TSP acct last year and it lowered my income and taxes and shows as "rollover" on my 1040. Any help would be appreciated.