Do you mean deforming a brush using Vertex Manipulation?
If this is the case, I'm no expert, but imo, VMing just takes practice to see what works and what doesn't. 2 good pointers from other's posts i've found useful:
1. Hammer does not like concave brushes or those that have sections 'turning inward'. Dandyli0n had a good analogy for this, which he said if you can imagine putting a rubberband around the brush, and it's not touching all sides, it's concave. concave = bad.
You can still make structues with concavity, like potholes, divits, bullet holes... just not individual brushes.
2. If you morph an object using VM and you get a 'invalid solid', you can clip the brush into pieces until the error is gone, or make the same thing by turing/flipping/rotating spikes and wedges.