Why would low complexity regions be linked with relaxed selection?
Low complexity regions have repeats in nucleotides ( or amino acids ).  E.g.  PPCDPPPPPKDKKKKDDGPP or AAATAAAAAAAATAAAAAAT.  This 2011 article compares 14 individual Plasmodium falciparum genomes and finds that these repeats are highly variable between individuals - some may be short AAAATAAAA  others may be longer AAAATAAAAAAAAATAAAAAAATAAAATAAAA....AATAA even to scores of bases. 

I think its not clear what these sequences do, but the mere fact that they can vary so much from one viable individual to another indicates that they are not strongly selective.  *If they were subject to selection, you would be able to see the difference in the variation in the health, fitness, and reproduction that these significant genetic changes have, but in this case the change in sequence would not change the properties of the sequence much if at all. * 

I think these are also related to copy number variable (CNV) regions.  which expand and contract in the genome easily even between identical twins.  (sorry for the indirect reference - like many SDN stories, the reference is totally wrong), but I remember this work...