A "chain whip" wraps around a sprocket and provides leverage, either to hold it relative to something which is connected — such as a wheel the sprocket is screwed on to, or another sprocket in a freewheel cluster, or a cassette fixing bolt.
For cassette removal, not much force is needed.
For sprocket removal and freewheel disassembly, high force may be needed, as sprocekts tend to "self-tighten" in use, and due to combinations of (a) oxidation, (b) lubricant wash-out, (c) galling, and (d) creep, a sprocket which is used a long time without removal may be very tight.
Here are two chain whips, shown roughly in positions they would be in for typical use:
Analysis of the forces is a little bit complicated because both chains on a tool participate in transmitting force. In this case, a perpendicular appears to be the right configuration to use, but that may not be true for some chain tool.
Note, in particular, that a handle's length necessarily a good indicator of leverage. For example, the black tool uses less than the full handle length, while on this sprocket the Bike Hand uses the sprocket to extend leverage beyond that provided by just the handle. Thus one should consider the "useful" handle length, rather than the measured length.
For example, here is a Campagnolo chain whip where the end of the wrench (with the short chain section) presses on the sprocket:
In the first picture, the black tool is a Wheels Manufacturing chain tool with about a 28 cm useful handle length. The silver one is a Bike Hand and shows about a 44 cm useful handle length, which is roughtly 1.5 times the leverage.
The Bike Hand tool also has a wider handle, which may allow higher hand forces without discomfort.
According to the Park Tools web site as of 2013/04, the SR-2.2 has a handle about 38 cm, and it appears from the design that is useful length. The handle is somewhat shorter than the Bike Hand, but the Park tool may be more widely available in some areas.
As of 2013/04, more information is available about the Bike Hand tools at http://www.bikehand.com look for the YC-505 and YC-506 tools; Park Tools and the SR-2.2 at http://www.parktool.com/product/sprocket-remover-chain-whip-SR-2-2 ; and Wheels Manufacturing and the chain whip at http://wheelsmfg.com/chain-whip-cog-removal-tool.html.)