the gemara at the end of the masechta is the source for not starting to talk before the avel, this can cause feelings of uncomfortableness especially with people that don't know the halacha.
rav elyashiv explains that the reason for this halacha is that you have to understand what exactly is most troubling to the avel so you can console him properly (a little psychology 101 from the great sage!). so being menachem an avel consists of 2 parts, being quiet and showing the avel this his loss is beyond words and description and you really have no way of consoling him (this is found rally in the prisha 393:3) the 2nd part is when you understand what really is troubling the avel you may try and talk to him about it.
here come the heter, since we don't console properly anyway, this 2nd part is a lost art (or for the most part anyway) and therefor he allows the following: one should come in and sit quietly for some time after which he is allowed to talk before the avel since this halacha is not stam an isur but a realistic way to be menachem and if you cannot achive this you may talk.
there is a story of rav pam coming to be menachem a family that lost a young girl and he was involved thru-out the ordeal, he came in and just sat there quietly and you saw the pain in his eyes, this was the family's greatest consolation. NOR OF SIMCHOS!