Explain compression to me

Can someone explain what compression is, I'm not sure I understand. Is it the folding at the hips? Which muscles are mostly working? How should it feel? Cues?

In the straddle up I have started to focus on folding mostly at the hip instead of using the abs mostly. Now it feels more in my hips and quads versus before when I felt it mostly in my abs. Is this correct feeling?

it will be quads, hip flexors and abs.  If you really want to feel it for straddle, sit in a pancake and put your arms in between your legs and try to lean over them a bit.  Then attempt to lift your heels and legs off the ground while maintaining toe point.

I know the frustration you are feeling brother! Haha, through my experiences it's a "balling up" sensation. Imagine sitting on your butt on the floor feet together or spread out to a straddle, place your hands on the floor as if you were going into an L or straddle L, and think about trying to get your feet off the floor while trying to keep your legs parallel to the floor....your butt would have to be behind your hands and your upper body kind of forward to bring your thighs to your chest essentially. Now go into the deepest pike stretch you can, trying to put your forehead on your shins, now push up into L trying to keep that position locked. Now that curled spine trying to keep the coil up is compression, and it's not easy to get strong at!

Yes, your hips are folding but that is not the movement. For an L-sit (easiest way for me to explain it) it all comes from you abs and your serratus muscles. Your hip flexors should be locking your legs at the hip angle, limited by your flexibility, while your abdominal compression rises everything off the floor. For the straddle ups it's sort of similar, but you are doing a lot all at one time. You are using hip flexors to help bring your legs up, at the same you are using abs to sit the chest up, and the compression is you trying to bring chest to thighs essentially while keeping your legs at that same angle.... Bring them up to the locked angle compress to get upper body to thighs....all at the same time! They are hard. I feel an incredible burn in the lower abs when I am getting good ROM with this movement.

Some moderators may be able to help more, and I hope I said all of that correctly. I feel like I understand it but it's a hard thing to get strong at for me.

I'm on my second go through of tuck ups (mn/pe1). i can do the reps easy but i'm not able to get my thighs as close to my chest as the video. is this a lack of compression? if so how do i correct? should i stay on tuck ups? will these ultimeately help with the compression? i tried straddle ups just to mess around and failed miserably - if i keep my legs straight mu torso only goes up about haqlf way.

I would stay on tuck ups and try improve the form & ROM over time.

You can do pike and pancake stretches, that will likely help a little bit.

Exercise patience too...

Stretch and then perform the exercise I mentioned above. This is how I improved my active flexibility. Also include some in your warmup everyday.

Daniel Burnham said:

it will be quads, hip flexors and abs.  If you really want to feel it for straddle, sit in a pancake and put your arms in between your legs and try to lean over them a bit.  Then attempt to lift your heels and legs off the ground while maintaining toe point.

I feel this very strongly in the quads more than the hip flexors. Is this normal? Perhaps this is my weak point with active flexibility, or lack of it

Anto.T said:

I feel this very strongly in the quads more than the hip flexors. Is this normal? Perhaps this is my weak point with active flexibility, or lack of it

It is normal enough.  People will feel it different places depending on where their weaknesses are.