The Sacrificial Fusion happens when the dominant person turns his/her body to energy then goes into the weaker person's body and change into a different person that has a few characteristics, but has a unique personality. The fusion is started by first giving energy then the dominant fusee turns his or her body into energy then transfers the energy into the weaker body.
After the fusion, the stronger fusee's power level acts as a multiplier to the weaker power level making the fused body extremely strong. The stronger the fusees are, the higher the multiplier is. Ex. Goku and Vegeta's power levels are in the millions when they do sacrificial fusion and become Gogito his power level goes from the billions to the sextillions.
SSJ4 Goku is about to fuse
Trivia
This fusion can be performed by any creature that has a power level over 10.