In some videos I have seen Colin Goldie and other whistle players cover the winway and blow sharply into the whistle before playing. What is the reason for this? It must have something to do with getting the whistle prepared but I wonder why? Thanks!
Blowing through an aluminum whistle with the window covered warms the whistle to reduce condensation clogging in the wind way.
… and blowing sharply helps clear any condensation already there (with the window covered).
As stated in both replies, it both helps to remedy and prevent clogging. I seem to get better results if I hold the whistle head firmly in my hand before giving it a blow through if it is an all metal whistle. Having the whistle a bit warm before blowing through will also prevent the initial blow through from depositing condensation (even if not in the head, the block or tube could have a condensation buildup from the initial blow-through, and having it warm prevents this).
Some people suck through the whistle to clear it if it clogs during playing, but I wouldn’t recommend it. I have a habit of doing that, and I sucked a plastic shaving down my throat from the inside of a sweetone 
luckily I think it was swallowed.. probably not sanitary to do this either.
Just keep playing. After the initial build up of mouth watering continued playing should diminish saliva.. it does for me.
Some people are wet blowers, while other people may be dry blowers.
From glancing at the title in a cursory manner my first thought was this https://www.youtube.com/watch?v=u7yUO3q8QfE 
Bob
I agree about holding the whistle head in your hand to warm it, or even putting it in your pocket for a while before you play,rather than blowing which can add condensation.
I’m a big fan of the suck technique to clear the windway if it clogs while you’re playing. Anything that’s in there came out of you so I doubt sanitation is a problem. With practice you can use this technique right in the middle of a tune which comes in handy in sessions or performances.
Do check the windway first for odd items like shards of plastic, as has been mentioned before.
snap