Some blocks can be stacked vertically while jumping with a sneaking.
(It's difficult to convey in text, so I'll post a video as well.)
(Fast-forwarding or changing file format due to capacity.)
For example, Full blocks, Half blocks, Stairs, Fence, Grass Pane, Iron Bars, Chorus Plant...
But there are blocks that can't do that.
For example, Anvil, Cobweb, Banners, Walls, Fence Gates Trap Doors, Carpets, Top Snows, Daylight Sensor, Chest, Brewing Stand, Lectern, Enchantment Table, Grindstone, End Portal, Hopper, Chain.