Why is the warm phase of the Pacific Decadal Oscillation associated with stronger El Niños?
According to the State Climate Office of North Carolina webpage Global Patterns - Pacific Decadal Oscillation (PDO), experts believe


  the PDO can intensify or diminish the impacts of ENSO according to its phase. If both ENSO and the PDO are in the same phase, it is believed that El Niño/La Niña impacts may be magnified. Conversely, if ENSO and the PDO are out of phase, it has been proposed that they may offset one another, preventing "true" ENSO impacts from occurring.


Essentially, almost analogous to constructive and destructive interference in sound.

Another relationship, from the University of Alaska, Fairbanks webpage The Pacific Decadal Oscillation also asserts that


  A positive, or warm phase PDO, produces climate and circulation patterns that are very similar to El Niño. Likewise, a negative, or cool phase PDO, produces climate and circulation patterns similar to La Niña (Gershunov and Barnett, 1998).


As can be seen in the image below:




Image source: Pacific Decadal Oscillation (PDO) Graphics

The consequences for forecasting are summed up by the NASA News web-article A Quirky El Niño as being the case


  If El Niño begins during a cool phase of the PDO, El Niño tends to be milder and less predictable. El Niños that come during a warm PDO are stronger and "better behaved, in the sense that we can predict their consequences


References:

Gershunov, A. and T.P. Barnett. Interdecadal modulation of ENSO teleconnections. Bulletin of the American Meteorological Society, 79(12): 2715-2725.