As CarolC says. But it does depend on your seedheads, and they are hard to push in the wet, and useless on long grass, and exhausting on big lawns.
There used to be big heavy ones that were two-man, one pulling on a harness. Very embarrassing if your mates came by. That's why they invented power-mowers.
Sorcha, they do have a box that hangs behind, on some types.

If you do find one, they cut like a pair of scissors, two sharp bledes slicing against each other. They sometimes need setting up, or sharpening if they've been allowed to get rusty.