The late Sandra Landy defined a game try as follows. You bid game and try to make it. More seriously you may be on the edge and require some help from partner. After 1 - 2 that will do you can bid another suit and this is called a trial bid. What sort of suit should you choose?. The answer is a suit that you need help, which eliminates short suits. After all if you have a singleton in a suit you don't need help in that suit, so always pick a four card suit, or possibly a three card suit where you need some help.  The trial bid suit is therefore a genuine source of side tricks, and you are asking your partner for help in the form of honours. What happens if you want to make an invitation and the suit that is bothering you is the trump suit, then bid three of the trump suit which in turn becomes a trial bid. A trial bid is always the bid of a new suit after suit agreement and responder should decline the invitation holding three cards in that suit. Kx is fine though. 

As it is referred to as a game try suit then trail bids are always bid at the three level allowing for sign off. In face this is not the case after 1♥  - 3♥  a bid of 4♣ / 4♦ would also be a trail bid, but as you have reached the game level it is a genuine suit with slam intentions. The principal remains the same though, a suit where help is required to go on.

In the example hand opposite a two level raise declarer assesses the chance of game using the loosing trick count, with a six loser hand he is certainly strong enough for a game try, being one loser better than minimum. North couldn't have a better hand with everyone of his honour points working, and has an easy bid to 4♠.

Finally, looking at the issue from the defence, it is probably always right to lead a trump in a trial suit auction. This is also true in a splinter auction. In this case this would be incorrect as you run the risk of conceding a natural trump trick. On the face of it, it looks as though you have 4 losers, but as long as you are careful with your entries to dummy, you win the first trick, cross to dummy with the K♠  and unblock the diamonds. Now you can return to hand with the A♠  and play off the diamonds, discarding your hearts.  You don't mind if the opponents use the outstanding trump on the last diamond, as you a merely exchanging one loser for another. A classic loser on loser play.

After a 2 level response from partner to your 1 level opening a rebid of 2NT shows 15-19 points and is forcing to game. This frees up a rebid of 3NT which does not mean 18-19 HCP old style. It shows 17+ with a fit in the suit bid by responder. Consider the following hand. 6NT has no play, but 6♣  makes despite the Q♠ being offside and the hearts breaking 4-2. To pull this off, after drawing two rounds of trumps, you try to ruff out the Q♠ . This fails. Next you test the hearts, they break 4-2 but luckily it is West who holds the out standing trump and the 4th heart. So because you have kept the 6♣  you can ruff the 4th heart and West has to under ruff the 13th trick.

As important as you think it is to show your strength, describing your shape should takes priority over points. Consider this hand which has a number of valuable points. 5-4-3-1 hands offer the opportunity to show the distribution to your partner as long as you resist the temptation to crowd the bidding. The first bid to take note of is North's response to the 1♠ opening. The only time you would bid 1NT with 3 spades is when you have no ruffing opportunities with the short hand i.e. 4-3-3-3 shape, and therefore it is safe to assume that when you hear 1NT from the other side of the table that your partner holds no more than 2 spades. 

Now opener rebids 2♦ , it is a mistake to consider a bid at the 3 level with fewer than 19 points, first of all you want to show your 3 card suit and secondly you want to give your partner room to show support for both the bid suits. By bidding 2♦ you achieve this at the slight risk of your partner passing, which they are quite entitled to do, but now North bids 2♠ , sometimes referred to as the forced preference. This conveys a lot of information. The two card suit and denial of a 4th Diamond. 

The big bonus is South can show his three card heart holding, and this allows North an easy route to 4♥. But there is much more than that. South can show a sixth spade by bidding 2♠ or a fifth ♦ buy rebidding diamonds. All of these shape showing bids are swept away by bids that squeeze out the bidding space with unnecessary jumps.

Finally a comment about the rule of 14. It's not my favourite yard stick, but when weighing up whether you hand is worth a two level response this might help. Add the number of your high card point to the number of cards in your longest suit and if it comes to 14 or more then you are fine to bid 2 over 1, otherwise take refuge in the dustbin 1NT. That's ok up to a point but a lot depends on how your high card points are distributed. In this case fabulous, although you don't know that until the third bid from South. 

Andrew Robson's Bridgecasts can not be recommended too highly. Every so often he comes away with a throw away comment, the best of which I will share here. Today's gem is when you have a choice of bids lean towards bidding at the bottom of the range rather than the top. Suppose you have a hand with 7 clubs and 10hcp. You could open this hand either 1♣ or 3♣. If you open 3♣ you are at the top of the range and your partner will pass more more often than not. You could easily miss a game, whereas if you open 1♣ then this is far less likely.

4441 hands in ACOL are always tricky, because they are most likely to make the rebid challenging. The recommend method is to use the TAPS system as in Hot (Hearts) and Cold (Clubs). So with a red suit singleton you open 1♣  and with a black suit singleton you open 1♥. In this hand South has a Red suit singleton and opens 1♣. The percentage chance of being dealt a 4441 hand is 3% although it feels more.