Practicing movements through tight spaces and under objects is relatively under explored part of parkour this is a shame as this type of practice is highly development of proprioception, fluidity and can provide a great challenge to strength and mobility. Capoeira and B-boy both offer a variety of movement strategies that can be adopted into parkour/movement over obstacles.
Tight space movement was a significant part of the practice of the founders but like training in nature has tended to be lost in translation compared to big jumps, kongs and cats, watch the rail work at end of this yamakasi video it still stand out as exceptional movement even compared to the best current traceurs if you watch the rest of the work you will see that jumping, vaults, cat leaps and wall runs and tic tacs have all been far more developed since.
Another early exponent of this type of movement was tim pisteur
Current traceurs who have gone deeply into this realm include Jesse Danger from the movement collective
and my old compadre at Parkour Visions Tyson Cecka
This has been an area I have been exploring lately so you check check out some of my recent videos for inspiration as well
Your mission find a set of rails an interesting tree or something similar and play with ways to find flow moving through tight spaces.