The gemara asks whether an eved can appoint a shliach to receive his gett, according to the opinion that he cannot receive his own gett since he is still owned by his master. The gemara concludes that although one cannot typically appoint a messenger to do something that they cannot do themselves, nevertheless an eved my appoint a messenger to receive his gett since he is included in the concept of receiving a gett - he can receive a gett for his friend who is an eved from his master. The mitzpeh eisan explains rashi, who explains the logic of the gemara. The primary issue is whether the inability of the eved to receive his own gett from his master is a weakness in the eved's ability to receive a gett, in which case he should not be able to appoint a messenger either. Or is it merely a technicality that the gett has to leave the master's reshus which we learn from teh gezeira shava from isha. The fact that an eved can receive a gett from another eved is evidence that it is not a lack in his ability, rather a technicality. So, the requirement of "only able to appoint a shliach for something you can do yourself" = one can only appoint a shliach for something that they have the power to do themselves (if not for a technicality that makes them unable to do it by himself).