proof.py

import bpy
import bmesh
import time

US = lambda: bpy.ops.wm.redraw_timer(type='DRAW_WIN_SWAP', iterations=1)

# START WITH CUBE or low vertex number mesh selected.

ao = bpy.context.active_object
oo = bpy.ops.object

oo.mode_set(mode='EDIT')

me = bmesh.from_edit_mesh(ao.data)
nverts = len(me.verts)
print(nverts)

for i in range(2):
    for iv in range(nverts):
        me.verts.ensure_lookup_table()
        me.verts[iv].select = True     #  light them up one at a time
        US() #update screen
        time.sleep(0.5)

    for iv in range(nverts):
        me.verts.ensure_lookup_table()
        me.verts[iv].select = False     #  turn them off one at a time
        US() #update screen
        time.sleep(0.5)

proof2.py

import bpy
import bmesh
import time

US = lambda: bpy.ops.wm.redraw_timer(type='DRAW_WIN_SWAP', iterations=1)

# START WITH CUBE or low vertex number mesh selected.

ao = bpy.context.active_object
oo = bpy.ops.object

oo.mode_set(mode='EDIT')

me = bmesh.from_edit_mesh(ao.data)
nverts = len(me.verts)
print(nverts)

for i in range(2):
    for b in [1, 0]:
        for v in me.verts:
            v.select = b
            US() #update screen
            time.sleep(0.5)

The lambda is in essence a single line function declaration, a shorthand for functions which really don't have much going on, but at the same you just clutter the code if you always pass the same arguments. In this case the lambda allows the execution of a function with pre-filled arguments.

The following are functional equivalents in this scenario.

def US():
    bpy.ops.wm.redraw_timer(type='DRAW_WIN_SWAP', iterations=1)

and

US = lambda: bpy.ops.wm.redraw_timer(type='DRAW_WIN_SWAP', iterations=1)

One could pass arguments to the lambda.. but then you might as well be using a function definition with default arguments if there's a set of arguments that would get passed most frequent

US = lambda t, i : bpy.ops.wm.redraw_timer(type=t, iterations=i)
US('DRAW_WIN_SWAP', 1)

#vs
def US(draw_type='DRAW_WIN_SWAP', iterations=1):
    bpy.ops.wm.redraw_timer(type=draw_type, iterations=iterations)

US()
US('DRAW_WIN_SWAP', 1)
US(iterations=2)