The trajectory is cotrolled by two key factors:
1. Dynamic Loft (loft of the club at impact): We manipulate dynamic loft by the amount of shaft lean that we hold on to through impact.
2. Angle of Apporach (A.O.A): Typically speaking, the steeper the angle of approach the lower the ball will go. For this reason we play the ball back in the stance for low shots.
These two factors trade off each other too. If you feel that the club is coming down too steeply because your getting a massive divot or the ball is spining too much, then you can shallow out the angle of apporach but must maintaine more shaft lean. This is how we keep it low with the woods where a steep A.O.A is less than optimal.
Ball Position and Aim
The more the ball goes back in our stance, the more the swing path is going to appraoch from the inside. For this reason, you will likely find that that your low shots are pushed right. You can account for this by pivoting harder into tha ball (turning your chest harder) to nuetralise the path or more simply; as you move the ball back, open the stance in parity.
Length of Swing
You will notice from the video that the low shots are best played with only a 3/4 back swing and the affect of holding shaft learn is the follow through ends quite abruptly too. Hence, the name, Punch shot'.
High Shots and Thinned Attempts
The high shot requirement of a shallow A.O.A means that you are likely to hit a lot of thinned shots. You will need to consider the lie before selcting this shot on the course. Certanly while learning the shot, it is worth giving yourself a favourable fluffy lie. You certainly know that you own this skill when you can play if from a tight lie.
These shots are relatively easy to play and the low shot in particular is a massive benifit to your game. In my experience, club golfers are reluctant to play them beceause they feel that a 'normal' shot is hard enough, therefore these must be too difficult. That isn't actually how it works; you may find that the low shots are very much within your grasp. You must find the convictoin to play them on the course so they become a trusted part of your game.