While I'm familiar with ukenagashi (also called nagashi uchi by the late T.K. Chiba shihan), I've never heard things phrased quite like your question (it's not really a block, to my way of thinking). Thinking about it, it's probably just referring to the usual three levels that pervade sword and stick work: jodan (high level: head), chudan (mid-level: torso), and gedan (low level: legs). You can slip by: early and high, maybe even dodging entirely, mid-timing and mid-level glancing deflection, or late and low, in which case it might resemble a block before following through.
edit: typo fix.
Last edited by jamesf : 03-04-2019 at 08:39 PM.