Throwblocks are blue blocks than can be grabbed by Mario in SMW. In order to perform a throwblock jump, a trick where you jump off the block while grabbing it, you must press Y + B on the exact same frame. You cannot hold Y before hand, both buttons must be pressed on the same frame. Doing a throw block jump with X + A will still work, but give you a normal jump instead of a spin jump.
You can also spin jump off of kicked throwblock. If a blue Koopa kicks the throw block, you can run up to it while it is going, gain p speed, then do multiple spin jumps off it. Mario's p speed is slightly faster than the kicked throw block, so you may occasionally need to tap left, then right to make sure you don't go ahead of it during long surfing sections. Learning when to start mario's run to make sure it matches up with the throw block, as well as managing the size of mario's jumps are the most important elements for this trick.