HCG will cause a large amount of test to be produced my the testes! Test aromatises into estrogen, so when there is a large amount of test there will be a decent amount of aromatisation, which is my it is good practice to run nolva with HCG! This is the way I understand anyways, if I am wrong please correct me!
I know for myself, who is very prone to estrogenic sides, one single shot of 500IU's made my nipples tender and also made me hold a significant amount of water!