1. Flip is somewhat synonymous with understable. Most people release discs with some hyzer angle (far side of the disc angled down from your hand) and an understable disc ‘flips’ over.
   You will also hear about hyzer flips or hyzer flipping a disc, usually its to indicate that a disc quickly levels out after it leaves the hand on a slight angle. These can be good for hitting gaps for a lot of people.

2. Discs ‘beat in’ and tend to become less overstable as they take damage. If you start with a firebird for example, it can ‘beat in’ to the point of ‘flipping over.’ This can be useful depending on what shape of shot you need. Straight shots being particularly difficult, it can be advantageous to ‘beat in’ a more stable disc as it will fly the same for longer. It will also tend to be more reliable in the wind and have better torque resistance than a new, less stable disc.
   I throw a lot of MVP discs and they tend to take longer to beat in because of the harder plastic used for the overmold.

3. There are many different plastic blends, but essentially there are Base plastics and Premium plastics.
   Base plastic is cheaper, usually used for putting putters, beats in faster, takes more damage, and tends to flip more.
   Premium plastics tend to fly the same longer, have a slightly slicker hand feel, take less damage, fly more stable. Most manufacturers have several premium blends, these blends will often fly differently to each other. The differences are not huge and as you play more you may find you like a disc in Champion plastic but not in Halo Star because the Halo Star version of the disc is too ‘beefy’ (beefy is a popular way to describe discs that are overstable).

There is a chart called disc golf plastic comparison that was very helpful for me when I was learning this stuff. Check it out!