there are two methods to modify weight of selected vertices.

first one will uniformly fill the vertices weight value. for example, the middle portion forearm bone would have 100% influence. 

whereas the tip and root of the bone is transition area to meet with other bones influence, I would use smooth weight and mothed two to fine tune the weight influence. since method two would modify weight value on a per vertex level.

this is especially useful for bending kneel area:

1. select a single mesh object and go to edit mode. (the addon works on single selected active object)

2. locate the addon tab enable "Vertex Weight Toggle"

3. there should be some vertex groups on the mesh after binding, for other scenarios, if there is not any vertex groups, add some and start to modify weight value with included tools.

4. any operators from the addon that can change the weight value will only work on selected vertices, if nothing is selected, it would do nothing.

first I recommend setup the shortcut according to your habit in addon preference.

With nothing selected or more than 1 vertex selected, you can move mouse cursor near a bone and use shortcut to run the operator. it will switch to the vertex group under the mouse cursor.

it is useful if you have already select some vertices to modify weight and you'd like to check different bone's influence.

under the hood, it samples what is under the mouse cursor and compare the weight of influenced vertex groups and set whichever is greatest as active vertex group.

for more complex area, select a single vertex and cycle all its influenced vertex groups, such as chest area.

Mirror weight will transfer weight value to selected vertices from relevant mirror parts.

for example, I have rigged a human character and done a simple binding. I fine tune the weights on left forearm and left leg. then I select all right side vertices and press mirror weight. right forearm and right leg would have similar mirrored weight.