He definitely said something important there.
Expressive playing in rock-like music styles does incorporate elements that a classical musician might also use, like dynamics and articulations, but it's actually much more, more about "having the blues". What that means is hard to define, so this is just my interpretation —
It means, for instance, bending notes to pitches that can't be exactly described in a score. It does, in fact, mean playing notes that you could not really define yourself: playing "from the heart" rather than from the mind. This may include notes that would classically be described as plainly wrong.
So, in contrast to what Victor said, correct phrasing is not always necessary or even desirable – you do have to find the right balance, but technically perfect may be not musically ideal.
To give a concrete example: consider the guitar solo in Since I've Been Loving You, by Led Zeppelin. This is by no means a technically perfect performance, the fast passages are sort of huddled together rather than phrased out properly. But that just gives it this incredibly emotional, desperately screaming character: I consider it one of the best soloes of all time.
Of course, there are more nuances of expression than just blues, those are typically closer to the classical expressive elements. Still, expressive guitar playing has a lot to do with letting go of from-the-head control over what your hands are doing; so it is not much good trying to practise this type of expression with specific exercises, and most certainly not with any "principles". You rather just have to practise so well that you can stop thinking about what you actually are playing, so that you can start letting the emotions "float into the music". The best way to get there is probably: lots of improvisation, preferredly with a band. When practising alone, avoid playalongs, better just jam around on your own.
That said: all this does, of course, not mean you should abandon technique and discipline. Sometimes an "expressive" part simply will sound outrightly wrong, and what sounds amazing when played in some climatic solo may be rather annoying in other songs. Most often, you're on the safe side when going the classical way as described by Victor: subtle and conscious modifications of pitch, attack and articulation. These are best practised on actual romantic pieces played on classical guitar, which I would definitely recommend over practising any parts from songs on electric guitar.
By the way, "conscious" modification will not necessarily be subtle: a prime example for excessive and greatly effective use of such elements is Jaqueline du Pré's Elgar Cello Concert, which is at least as expressive as any blues guitar solo but still considered close to technical perfection.