understand the details of einsum application for two tensors

We have two tensors:

 a = np.arange(8.).reshape(4,2,1) b = np.arange(16.).reshape(2,4,2)

We are going to impliment

np.einsum('ijk,jil->kl', a, b)

Although we could obtain its results, we were persist to understand the process details about the summation over tensors element.

Firstly we know how

np.einsum('jil', b)

changes the elements' orders of b tensor.

but we cannot understand how np.einsum('ijk,jil->kl', a, b) combine (sums) tensors elements.

For tracking the process we used strings:

aa=[[['e'],  ['r']], [['t'],  ['y']], [['u'],  ['o']], [['p'],  ['q']]]

and

bb=[[[ 'x', 'c'],  [ 'v' , 'n'],  [ 'm',  'h'],  [ 'f' , 'd']], [[ 's',  'w'],  [ 'a','z'],  ['j', 'k'],  ['l', 'b']]]

Because we wanted to see how different elements combine for obtaining np.einsum('ijk,jil->kl', aa, bb)!

However np.einsum('jil', bb) works correctly but it did not show me the details of summation over elements.