Soto uke is performed by bringing one hand to the ear, keeping the elbow raised, and sweeping the arm down and out towards the center of the body.
The preparatory step of raising the arm gives the block more momentum, making it somewhat painful to encounter.
A different application involves extending the non-blocking hand in front before performing the block.
The motion termed the block, then, is actually meant to break the arm, which would explain the necessity of the extra momentum.
[6][7][8] The above designations are particular to Shotokan and some other schools, and are backwards from other systems (Wado-ryu, Doshinkan, Uechi-Ryu, etc.)