The first tool here allows you to R&R valve springs without removing the cylinder head, as if you were replacing a broken spring or installing valve seals. When using this tool you need to have some way to hold the valve in place from underneath. This can be done with air pressure in the cylinder via a fitting in the spark plug port. It might also be done by stuffing some small rope into the cylinder and bringing the piston up to push against the valve head.

There are a number of similar tools on the market in the $12 to $30 range. Some may be cam and lever operated for fast action. One thing to watch for is the length of the short fingers on the ends of the hooks, as some of them are too short to catch the inner spring of a dual valve spring setup. This particular model will work with dual springs, but you may need to put a screw type hose clamp around the legs of the tool to hold it securely in place during use.

The second tool here is much easier to use if you have the cylinder head on the workbench. The forked end catches the valve spring cap while the peg end presses on the valve head. The actuator has a toggle mechanism that works with a single squeeze of the hand lever, and there is a latch to hold it in the pinched position to free up your hands and let you relax once the valve spring is compressed. These can be found in the $20 to $30 range. Beware that some of the cheaper ones may be particularly flimsey, perhaps good for small engines but not well suited for use on your car engine.

A trick with any of these tools is to put a little force on the tool, and then give the spring cap a little tap with a hammer to break the valve locks (cotters) loose. Trying to change a valve spring without a spring compressor tool can be a real case of masochism.