Can't say anything definite about Yrden, Axii or Quen, but here's what I observed with Igni and Aard.As long as you don't wait for the sign to fully power up (Geralt makes a sweeping hand gesture and crouches a bit, and his left hand glows with either lightning or fire), you get the sign's "base" form, with strength adjusted for power-up. So, both an instant Igni and halfway-powered Igni produce a "wave of fire" effect, but the halfway-powered Igni will inflict more damage.Fully powered Signs always produce the "upgraded" effect, i.e. Igni creates a long-distance fireball, Aard creates a 360 degrees shockwave, etc.