[CFHollister]

The "sliding" process that is discribed here is what linguists call a diphthong, or coarticulation of two or more vowel sounds. Many "long vowels" in English end up as diphthongs even if we don't realize it. Some end up being written with two vowels; but others that we think are "pure vowels" really aren't in many English dialects:
long i as in hight
long a as in late
oi as in boil
ow as in cow
and sometimes even long o as in hope which ever so slightly "slides" from oh to oooh. If you can feel your lips tightening while you say a long o, then you know you are doing it.

Other langauges (including French) tend to use more of what are called "pure vowels," vowels which do not change in articularion as you say them. This is particularly true in what English speakers think of a long a. I kow this post was mostly about English, but recognizing what you do instinctively in you native language helps you consciously not do those things when speaking another language. Learning proper French pronunciation also includes English accent reduction.

Edit: The so-called "semi-vowels," w and y, can also be seen as a form of diphthongization that has been standardized where w represents a diphthong starting on oooh and ending on the vowel that is written after it and y represents a diphthong that starts on eeeh and ends on the vowel written after it.