The bottom line is; flexibility comes from being able to relax your muscles into a stretch NOT from contracting and pushing the stretch further.
That said, it doesn’t mean you can never “push”, however pushing in a stretch is a very advanced skill and shouldn’t be practiced by anyone who cannot isolate the contraction of 1 muscle while at the same time relaxing an other.
The biggest mistake keen beginners make is wanting the flexibility so bad that they’re willing to “do whatever it takes” to get more flexible. What usually happens in this case is they attempt to stretch on their own or take a class with too many people so that the instructor has no 1 on 1 time with them (so they don’t stick out) and “get in the zone” and contract every single muscle trying to “push” deeper and deeper into the stretch and leave the stretching class feeling like they’ve just run a marathon. Although the effort is great, this individual is more likely to injure themselves and set themselves back rather than making safe gains and improving their training capacity.
Think about it this way; if you have a rock (aka a contracted muscle) and you try as hard as you can to try and pull it and make it longer the only things that will happen are 1 nothing, and 2 enough force is placed on it that it cracks or tears in half. Now, if you have a ball of play doh (aka a relaxed muscle) and you let it warm up from the blood flow going to your hands to warm up the dough, and you apply gentle pressure it will gradually lengthen while little force applied other than gravity.
Now take this image, chose a stretch that is fairly easy and you understand well. Close your eyes and try imagining the muscle that you’re focused on stretching in that particular pose and think of it first as a rock, and then as play doh. Breath and relax!