I'm pretty sure that everyone and their mom knows what this core does, but here is an explanation just in case you've been living under a rock since ORAS has been released lol. Metagross and SpecsDeo form a potent wallbreaking core that can really destroy a lot of teams pretty easily if you play well. Keldeo's most common answer in OU is probably the Lati twins, and pursuit mmeta can switch into them with ease and pursuit trap them, paving the way for Keldeo to sweep through many offensive teams. Additionally, Keldeo has a lot of trouble breaking through fat fairy types like Azumarill, Clefable, and Mega Altaria, so Metagross is a pretty solid partner by virtue of its steel typing. Additionally, bulky grass types that are counters to Keldeo like Venusaur and Amooonguss all get destroyed by metagross, and the same extends to poison as well as flying types (bar tflame). On the flip side, Mega Metagross has many answers that are taken care of by Keldeo. Defensive ground types wall most meta sets, but they are owned by Scald/Hydro Pump. Dark types can really hurt meta with their STAB moves, and Keldeo lights them up with secret sword. Steel types are severely crippled by scald, and they aren't exactly eating up specs scald in the first place. Keldeo has HP Electric for this core specifically so that it can lure in and 2HKO bulky waters that trouble both members of the core otherwise like Gyarados (normal), Slowbro, Tentacruel, and others. Aside from that, there isn't much else to say for this core aside from the fact that's it's really good.