From Populous Wiki
The Blast Trick is essentially a glitch and is considered as such by some people, while others insist that it is more of a trick. According to Stuart Whyte the Blast Trick is an unintended bug. There has been a huge discussion among the popre community, whether to remove the Blast Trick. In the new Beta there is an option you can tick before starting the game to remove the Blast Trick.
The basic idea behind performing this glitch is, to not receive any fall damage at all. By using the Blast Trick correctly you only receive damage of the blast itself.
Mechanics behind the Blast Trick
In the video below one can see, that the Blast Trick does not work in every direction. The camera direction is set on 0.
Looking at the video, one can clearly see that the Blast Trick is working perfectly fine in directions 1, 2, 3 and the diagonal directions. If your shaman is walking towards direction 0, you cannot perform the Blast Trick - the reason for this is probably that the Shaman is flying higher in direction 0, than in the other directions. To perform the Blast Trick your shaman needs to be walking, because a follower/shaman is always blasted in direction 0 by default.
Map directions, when camera is set on 0 (press 0 in game)
In order to not receive fall damage, you need to time the moment when you cast a spell. Shortly (~100ms) after the blast of an enemy shaman lands on your shaman, you need to cast a spell. The more you are training to perform the Blast Trick, the easier it gets to time the moment to cast.
Timing when to cast a spell - basically directly after the blast effect lands on your shaman.
This glitch can also be used to avoid receiving fall damage, when be being shot by Firewarriors - usually 2-3 Firewarriors can blast you as high, as a normal blast, so this is the perfect number to perform the trick. It also works with only one Firewarrior, but the fall damage you receive is not high anyways. Four or more Firewarriors make your shaman fly too high though, so the Blast Trick does not work anymore (similar problem as being blasted in direction 0).