Just think of sliding your hand and arm forwards and down into the water ahead of you, as if reaching forwards over the hood of a VW Beetle, having swum forwards through the front windscreen(?)
When you reach the front fender, at full or suitable extension (reach), your fingers will be slightly lower than your wrist, your wrist slightly lower than your elbow, your elbow slightly lower than your shoulder.
When you then start your catch (attempt to hold onto the front fender and pull yourself forwards), if you can keep your elbow up above the hood throughout the pull, (or hold more accurately), then your elbow will remain high throughout your catch and hold on the water, giving your a more efficient catch.
But if your elbow drops onto the hood as you attempt to catch and hold the water, to pull yourself forwards beyond the fender, (through the water), then you lose the forearms use to hold onto the water vertically, instead dropping the elbow and putting the forearm horizontal to the surface of the water, and only catching with your hand, or not all.
Make sense?
If that doesn’t give you a picture, just think of what someone else said about paddling a surf board.
Elbow high throughout and the forearm and hand vertical at the start of the pull, and remaining vertical to pull you forward and catch the wave this time.
Contrast that with just resting your whole forearm in and parallel to the surface of the water alongside your board, and then just skimming the forearm and hand back to you side in an attempt to pull yourself forwards. It’s not going to happen is it as you are just skidding or slipping back through the water with your hand and forearm.
Get those forearms vertical!
But don’t be surprised if you apply some oomph and they drop. No need initially for acceleration or brute force, just find the VW hood position through repetition and slowly you’ll start to catch and hold more water on every stroke, slipping less and less.
If you sense that your apparent arm speed under your body after you have initiated your stroke, matches the speed of your body forwards past that arm, then you have some efficiency to your catch, but if you sense that your leading arm strokes back under you without a matching forward momentum of your body, then you are slipping, not holding as well.
And this usually happens when you breathe rather than on non-breathing strokes… Sometimes for a reason, but usually out of habit if you have cleaned up any head-lifting issues when breathing etc.
Have fun!