In the second part of the game the choice goes from "play any card" to "follow suit if possible & must win trick if possible". I can accept moving to "follow suit if possible" but why the second restriction?
1. A lot of old really old trick taking games like Tarot have this restriction.
2. I've played a lot of Schnapsen, and the main strategic -- and most exciting -- choice is deciding when to trigger the second phase. That second restriction of having to win the trick if possible gives you a bit of an edge over your adversary as it enables you to drain trumps from your adversary more quickly. So it gives you an additional incentive to trigger this second phase.