If we speak about relaxed punching, real methods of the internal arts and Tai Chi, then I like bag practice very much. Practicing on a bag or similar is important if you want to learn how to keep relaxing upon impact, through the whole process of the punch. I like deep penetrating, heavy, blows with whole body coordinated movement. But as the Mrs doesn’t allow me to have a bag at home
... I usually practice punches against a kicking protection.
You really need some sturdy surface to practice this kind of striking on. If you believe that you are going to be able to deliver a good relaxed punch without testing your balance and alignment against some kind of surface, you will be fooling yourself. It's very hard to understand how your body alignment and balance will become affected upon impact if you don't actually practice while receiving that impact and the returning force that comes with it. If you want to learn how to relax through the target, alignment becomes a big issue.
There are many different methods to try, experiment with, coordinate together, and to have fun with. You can punch with "closing", or on "open", by utilising "falling" or "raising". You can practice to strike on exhale or inhale. If inhale, you punch using "open" principle, the whole torso of the body expands from the ribs. As you don't tense your breath, there's no harm or negative impact on the lungs.
It’s also important to practice both whole body movement from the beginning with“body pushes” the fist, and also "fist leading" and body follow up. Here, the punch looks disconnected, like moving by itself, but after the fist has travelled a bit, the body arranges itself so that upon impact, the whole body is coordinated and has the best angle and alignment to support the fist.
You can try to lead the fist with the foot, kua, by turning the waist or centreline. You can also try different things like punching while keeping the center completely straight, moving in while keeping your center straight, or try to coordinate raising or falling movements together with horizontal movements of the body and waist.
Every different way to coordinate the body will affect your alignment upon impact in different ways, which means that you need to adjust the stance/posture and distance in different ways to have the best structure ad support as possible when the fist meets the bag/surface.
From my own experience, being able to really relax on impact, and all the way into the target, is a skill that takes a whole lot of time to develop. But this is what you need to learn if you want to be able to express real "jin" through the punches. More than twenty years ago, when I first started to really practice relaxed, genuine "IMA" and Tai Chi punching, in a methodologically structured manner, there were always different places in the body that I tended to tense up. It can be breath/chest, Neck/jaws, shoulders. At first, the whole punching process felt a bit weird, sometimes unnatural. But the more instructions I received and the more my teacher enlightened me on every little detail I did wrong, the more logical the whole process of relaxed striking became. It took many years to get rid of the impulse to "want to feel strength" and to get rid of the tensions that occurred through the punching process.